Every English verb has five different forms, but only two of the forms have a tense feature. The tensed forms are indicated with a morphosyntactic feature, either [+past] or [-past]. The form that a verb in the V-head position takes depends on what tense feature is in the T-head position, among other things.
But there are some quirks of English that can make things confusing:
Is it bare or [-past]?
For just about every verb, the [-past] form is recognizable in the 3rd-person-singular form (she eats/walks/sings/takes). The 1st & 2nd-person forms (I eat/walk/sing/take and You eat/walk/sing/take) look just like the bare form (eat/walk/sing/take).
If you’re looking at a verb and can’t tell if it’s in the bare form or the [-past] form, give it a 3rd-person subject and then look for the –s morpheme:
I want to visit Saskatoon. (bare or [-past]? Can’t tell: they’re ambiguous)
She wants to visit Saskatoon. (wants is [-past], visit is bare)
Is it [+past] or past participle?
For many English verbs, the [+past] form (She bought a donut) and the past participle form (She has bought a donut) are the same.
If you’re looking at a verb and can’t tell if it’s in the [+past] form or the past participle form, try replacing it with the verb eat:
She bought a donut after she had walked the dog. ( [+past] or past participle? Can’t tell: they’re ambiguous)
She ate a donut after she had eaten the dog. (Silly, but grammatical)
The form ate is [+past], while eaten is past participle, so we can conclude that, in that sentence, bought is [+past] while walked is the past participle)
What about auxiliaries?
The modal auxiliaries never change their form: they occupy the T-head position in their own right.
The non-modal auxiliaries, like main verbs, change their form depending on what tense feature is in the T-head position, among other things.