Agreement of tenses is what holds a sentence together across its different clauses (when a sentence has two or more clauses). The sequence of the actions (which happens first, which happens second, etc.) determines the correct verb tense.
Actions across the main and dependent clauses can occur at the same time (simultaneous) or one may precede the other.
We will review two possibilities for agreement with the indicative (present and past); we will then review three possibilities for agreement with the subjunctive (present, past, conditional).