Usage – perfect tenses

Yet
*Yet is always negative.

subj. + have + participle + yet
I haven’t eaten dinner yet.

Or

subj. + have + adverb + infinitive
I have yet to eat dinner (this is very formal and not a perfect tense structure)

Just, already

subj. + have + adverb + participle
I have just eaten dinner.
I have already eaten dinner.