What makes a chair conformation stable?
Physics, thermodynamics, geometry, God?
This is certainly a situation where you should get a set of molecular models, and play around with them. You know that the general formula of an alkane is
The formula for cyclopropane is
Expand the ring to 6 members, i.e. cyclohexane, and the natural bond angles of
When the ring flips conformation, the axial substitutent swaps positions with the equatorial substituent. If we put bulky substituents on the ring,
How to remember all this? Well the best advice is to repeat what I said at the start. Get a set of molecular models, and model the interactions. Is a chair conformation easy to flip? Does a planar ring introduce strain? And how do you represent the 3D structure on the 2D page? This is NOT trivial, and requires some practice to do so unambiguously.
Such molecular models are always permitted in examinations. Your prof (who will certainly have such a set of models on his/her desk) will recommend a set to get. You might even find them in the library.