The perfect and the imperfect
Biblical verbs mark aspect more than calendar tense. Perfect (qatal) and imperfect (yiqtol) need context, and often a vav.
Perfect (qatal)
Suffixed conjugation. In narrative it often reports a complete situation ("he said," "he made"). In other genres it can be stative ("he is great") or performative. Person, gender, and number sit on the end of the stem.
he guarded / he has guarded
she guarded
you (ms) guarded
Imperfect (yiqtol)
Prefixed conjugation. It can be future, habitual, modal, or the raw material of wayyiqtol. Prefixes א ת י נ mark person. Do not glue "imperfect = future" as a rule.
he will guard; he would guard
you (ms) / she will guard
I will guard
Jussive and cohortative
Short imperfects (jussive) and lengthened first-person forms (cohortative) carry wish and command, especially with אַל or וְ. They are easy to miss in an unpointed text.
let there be
Gen 1:3do not fear
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Vav-conversive
A prefixed ו can flip the usual force of a perfect or imperfect. Narrative past in the Torah is mostly this form.
The binyanim
Hebrew verbs inflect in stems (binyanim) that shift voice and intensity on the same root.
The participle
A verbal adjective: "guarding," "a guard," sometimes a present-time situation in narrative.