We need to know about basic topics such as Electronegativity and Bonding, which then leads to an appreciation of Bond Strength and how to spot weak bonds that are likely to be broken in a mechanism.

Understanding Acids and Bases and the pH of the reaction medium will be key. This dictates what is allowed and present in a reaction and what is not.

Being comfortable with where charge prefers to be (negative in anions, positive in carbocations) will help to explain a lot about intermediate structures and product outcomes.

If we are solid on these basics and know that only a limited number of things can happen as bonds form and break, the bigger mechanisms such as the Claisen condensation shown here become much more manageable.