Dont shift, or change, tenses when two or more events occur at the same time. EG INCORRECT: The dogs caught sight of the cat, and at the same time chase it. [The tense needlessly shifts from the past to the present.]

CORRECT: The dogs caught sight of the cat, and at the same time chased it. [Now it is clear that both events happened at nearly the same time.]

Do shift tenses to show that one event precedes or follows another. EG. INCORRECT: By the time the omelet was ready, I set the table. [The two past-tense verbs give the mistaken impression that both events-the omelets cooking and the setting of the table-happened at the same time.]

CORRECT: By the time the omelet was ready, I had set the table. [The shift from the past tense (was) to the past perfect tense (had set) clearly shows that setting the table happened before the omelet was cooked.]

Keep a statement about a general truth in the present tense even if other verb are in the past tense. EG. We learned that water expands when it freezes.