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.