Welcome, E x.
Here's a rule of thumb: if there are two verbs in a row, the first verb is the main verb (finite) and the second verb is non-finite.
EX: I walked. <1 verb: "walked" is a finite verb>
EX: I had walked. <2 verbs: "walked" is a non-finite verb; it's a past participle>
Finite verbs agree in person and number with the subject,
EX: She walks. <present tense>
Non-finite verbs are not connected to the subject. They are objects of the main verb,
EX: It has to work.
<modal + non-finite verb; "work" is connected to the verb "has to", not to the subject>
EX: I am supposed to go.
<be + adjective + non-finite verb; "to go" is connected to "be supposed", not to the subject>
Again, when in doubt, count the number of verbs. If there are two verbs, the first agrees in person and number with the subject (finite), the second does not (non-finite).