light is said to have a dual nature (particle and wave).

1. why does it behave like a particle when we dont observe it but like a wave when we observe it (Copenhagen interpretation of quantum mechanics)?

2. if photon is known to be fundamental particle of light..how come it has zero mass?

3. einsteins says nothing can move faster than light (c).but why does tachyons go faster?

I am not a physics expert by the longest shot but let me answer the third question to the best of my ability...
General Relativity only deals with the larger bodies but when we try to apply it to subatomic particle, it breaks down. Quantum mechanics is much more accurate in defining the movements of these small particles. In this subatomic world, everything is governed by the law of probability, so it might be able sometime for some particle to travel more than speed of light.

Q.3

Draw a graph, with momentum (p) on the x-axis, and energy (E) on the y-axis.  Then draw the "light cone", two lines with the equations E = +/- p.  This divides our 1+1 dimensional space-time into two regions.  Above and below are the "timelike" quadrants, and to the left and right are the "spacelike" quadrants.

Now the fundamental fact of relativity is that E² - p² = m².  (Let's take c=1 for the rest of the discussion.)  For any non-zero value of m (mass), this is a hyperbola with branches in the timelike regions.  It passes through the point (p,E) = (0,m), where the particle is at rest.  Any particle with mass m is constrained to move on the upper branch of this hyperbola.  (Otherwise, it is "off-shell", a term you hear in association with virtual particles - but that's another topic.)  For massless particles, E² = p², and the particle moves on the light-cone.

These two cases are given the names tardyon (or bradyon in more modern usage) and luxon, for "slow particle" and "light particle".  Tachyon is the name given to the supposed "fast particle" which would move with v>c. (Tachyons were first introduced into physics by Gerald Feinberg, in his seminal paper "On the possibility of faster-than-light particles" [Phys.Rev. v.159, pp.1089--1105 (1967)]).

Now another familiar relativistic equation is E = m*[1-(v/c)²]^-1/2.  Tachyons (if they exist) have v > c.  This means that E is imaginary!  Well, what if we take the rest mass m, and take it to be imaginary?  Then E is negative real, and E² - p² = m² < 0.  Or, p² - E² = M², where M is real.  This is a hyperbola with branches in the spacelike region of spacetime.  The energy and momentum of a tachyon must satisfy this relation.

You can now deduce many interesting properties of tachyons.  For example, they accelerate (p goes up) if they lose energy (E goes down).  Furthermore, a zero-energy tachyon is "transcendent", or infinitely fast.  This has profound consequences.  For example, let's say that there were electrically charged tachyons.  Since they would move faster than the speed of light in the vacuum, they should produce Cherenkov radiation.  This would lower their energy, causing them to accelerate more!  In other words, charged tachyons would probably lead to a runaway reaction releasing an arbitrarily large amount of energy.  This suggests that coming up with a sensible theory of anything except free (noninteracting) tachyons is likely to be difficult.  Heuristically, the problem is that we can get spontaneous creation of tachyon-antitachyon pairs, then do a runaway reaction, making the vacuum unstable.  To treat this precisely requires quantum field theory, which gets complicated.  It is not easy to summarize results here.  However, one reasonably modern reference is Tachyons, Monopoles, and Related Topics, E. Recami, ed. (North-Holland, Amsterdam, 1978).

As to proving the existence of tachyons, one basically has to discover a particle interaction which can *only* be explained by the presence of one or more tachyons. Some theoreticians argue that if tachyons exist, the universe could be filled with them but they interact so weakly with ordinary matter that we can't detect them. Physicists have searched through some experimental records and so far none of the high-energy accelerator labs have detected an interaction which can *only* be explained by tachyons. This means that tachyons must be far more weakly interacting than neutrinos. If they do exist, tachyons would be extremely difficult to utilize under our current understanding of physics.

2.

Does the photon have mass?  After all, it has energy and energy is equivalent to mass.

Photons are traditionally said to be massless.  This is a figure of speech that physicists use to describe something about how a photon's particle-like properties are described by the language of special relativity.

The logic can be constructed in many ways, and the following is one such.  Take an isolated system (called a "particle") and accelerate it to some velocity v (a vector).  Newton defined the "momentum" p of this particle (also a vector), such that p behaves in a simple way when the particle is accelerated, or when it's involved in a collision.  For this simple behaviour to hold, it turns out that p must be proportional to v.  The proportionality constant is called the particle's "mass" m, so that p = mv.

In special relativity, it turns out that we are still able to define a particle's momentum p such that it behaves in well-defined ways that are an extension of the newtonian case.  Although p and v still point in the same direction, it turns out that they are no longer proportional; the best we can do is relate them via the particle's "relativistic mass" m_rel.  Thus

           p = m_relv .

When the particle is at rest, its relativistic mass has a minimum value called the "rest mass" m_rest.  The rest mass is always the same for the same type of particle.  For example, all protons, electrons, and neutrons have the same rest mass; it's something that can be looked up in a table.  As the particle is accelerated to ever higher speeds, its relativistic mass increases without limit.

It also turns out that in special relativity, we are able to define the concept of "energy" E, such that E has simple and well-defined properties just like those it has in newtonian mechanics.  When a particle has been accelerated so that it has some momentum p (the length of the vector p) and relativistic mass m_rel, then its energy E turns out to be given by

           E = m_relc² ,   and also    E² = p²c² + m²_restc⁴ .           (1)

There are two interesting cases of this last equation:

1. If the particle is at rest, then p = 0, and E = m_restc².
   
2. If we set the rest mass equal to zero (regardless of whether or not that's a reasonable thing to do), then E = pc.

In classical electromagnetic theory, light turns out to have energy E and momentum p, and these happen to be related by E = pc.  Quantum mechanics introduces the idea that light can be viewed as a collection of "particles": photons.  Even though these photons cannot be brought to rest, and so the idea of rest mass doesn't really apply to them, we can certainly bring these "particles" of light into the fold of equation (1) by just considering them to have no rest mass.  That way, equation (1) gives the correct expression for light, E = pc, and no harm has been done.  Equation (1) is now able to be applied to particles of matter and "particles" of light.  It can now be used as a fully general equation, and that makes it very useful.

Q 1. afai khojnus!!!

All of your questions were the biggest issues for physicists before Richard Feynman.
1. It is not the proper way of asking question. The wave-particle duality itself is not the right theory you must consider with. It is better to go this website and listen the lectures given by Feynman in New Zea land: http://www.vega.org.uk/video/subseries/8. If you can't find answer from his lecture, nobody will help to you.
2. Photon is not the fundamental particle of light. In physics language, there is something called quasi-particle or excitation. This is a convention so it is not like electron or...... For zero mass you have to stick with general theory of relativity.
3.  even wiki answer is already good enough for this question that what people are replying to you.
BTW, I am not a physicist either but there are thousands of unanswered questions in physics even now and any theory at particular era will work until people find another theory better than the previous one.
You can also read the book written by Stephen Hawking (about time) just for fun (if you enjoy it!).

what particle is gravity made from and what causes it?