Infinitive or perfect past tense

Infinitive or perfect past tense
 

pero acabus de llegar

If I were composing this I would be tempted to use the perfect past rather than the infinitive, I guess if the person has arrived then it it technically in the present or close to it.

When is this construction called for?

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acabas is present simple indicative

yo acabo
tú acabas
él acaba
....

acabas de llegar ---> acabar de + infinitivo ---> it's a verbal periphrasis meaning "have + just + infinitive" or "just + past simple"

acabas de llegar = You've just arrived
acabo de comer = I just ate

So when you say "have +" Do you mean this may have been preceded by haber?

haber + acabar de = infinitivo

Is there a distinction in Spanish then such that if you have arrived, then that is present tense (you are now here) but if if you just arrived that was talking about the time in the past when you arrived?

acabar de + infinitivo= to have just done something (idiomatic)

The auxiliary verb haber isn't needed in this case, since the idiomatic expression has that meaning when translated to English.

acabo de comer = I've just eaten