Decisions, decisions. SN1 or SN2 substitution? E1 or E2 elimination? How should we decide when to propose a particular mechanism? Simply put, many a times these reactions are in competition with one another. To make a decision, we need to consider the reaction conditions because the base/nucleophile and solvent play a critical role in the mechanism.
I have made a small flowchart-style guide to help students in this respect. This guide is meant for reference purposes, and does not provide detailed explanations as to why a particular mechanism is favoured. Students should consult a textbook (I recommend Klein's Organic Chemistry 3rd Edition) for further reading.