Weak verbs
Roots with gutturals, nun, yod, vav, or repeated letters lose or change consonants. They are the majority of common verbs.
Why they look irregular
A "strong" verb keeps three root letters visible. Weak verbs hide one: נָתַן drops nun, בּוֹא is a middle-vowel root, רָאָה ends in he, הָלַךְ behaves partly like a yod-first verb. Frequency lists are full of them, so you meet them on day one.
he gave (I-nun)
he walked
he saw (III-he)
to come (hollow / middle vowel)
Wayyiqtol makes it louder
וַיֹּאמֶר, וַיַּרְא, וַיֵּלֶךְ, וַיִּתֵּן are the narrative forms you will see more than the textbook qatal. Parse from the prefix and the remaining root letters, then check a chart.
and he went
and he gave
and he saw
A practical order
I-guttural and III-he first (אמר, ראה, עשה), then hollow (בוא, שים, מות), then I-yod/I-vav (ישב, ידע, ילד), then I-nun (נתן, נפל, נשא). Do not postpone them until after "all the strong verbs."
Read next
The binyanim
Hebrew verbs inflect in stems (binyanim) that shift voice and intensity on the same root.
The perfect and the imperfect
Biblical verbs mark aspect more than calendar tense. Perfect (qatal) and imperfect (yiqtol) need context, and often a vav.
The 100 most common words
A short list does most of the work in Biblical Hebrew. Particles and a handful of verbs open whole chapters.