A tense is:
A property of verbs in which the time of the action or state, as well as its continuance or completion, is indicated or expressed.
So when talking of tense we actually refer to the verb of the sentence. But as we all know only finite verbs have tense. Some verb forms such as infinitive, participle and gerund don't have any tense.
Only finite verbs have tenses regardless of the mood: indicative, subjunctive, imperative and interrogative (here I refer to the principal moods only).
Now, let's take your examples one at a time:
- You seem to be smart.
This is indicative mood, present tense.
- I am hungry.
Same as #1
This is an interjection, so there's no verb here.
- Close the door!
This is imperative mood, present tense.
- How are you?
This is interrogative mood, present tense.