The binyanim
Hebrew verbs inflect in stems (binyanim) that shift voice and intensity on the same root.
The usual set
Qal (simple), Niphal (often passive or reflexive of qal), Piel and Pual (factitive or intensive, with Pual as passive), Hiphil and Hophal (causative, with Hophal as passive), Hithpael (reflexive). Not every root appears in every stem. Names are traditional; meaning is per root.
he guarded (qal)
he was guarded / he watched himself (niphal)
he destroyed (hiphil of שמד)
he prayed (hithpael of פלל)
How to recognize them
Niphal shows נִ in the perfect. Piel has a dagesh in the middle root letter in typical strong verbs. Hiphil often has הִ and a yod in the imperfect. Pointing is the clue; unpointed text needs more context.
Do not over-translate the label
"Piel equals intensive" fails on many verbs. Learn the stem as a dictionary fact for that root, then let the verse decide the English.
Read next
The perfect and the imperfect
Biblical verbs mark aspect more than calendar tense. Perfect (qatal) and imperfect (yiqtol) need context, and often a vav.
Weak verbs
Roots with gutturals, nun, yod, vav, or repeated letters lose or change consonants. They are the majority of common verbs.
The participle
A verbal adjective: "guarding," "a guard," sometimes a present-time situation in narrative.