Learn Italian: Credo che questa impronta sia tua. - I believe that this footprint is yours.

Sia is the present subjunctive of essere (to be). You must use the subjunctive here because the sentence starts with Credo che (I believe that). Verbs expressing opinion, belief, or doubt trigger the subjunctive in the following clause because they express subjective thoughts rather than objective facts.

Why is it just tua and not la tua?

Normally, Italian possessives require a definite article, as in la tua casa. However, when a possessive comes directly after the verb essere to state simple ownership, the article is usually dropped. If you said sia la tua, it would specifically mean 'is the yours' (the one belonging to you out of a specific group).

The subjunctive sia is the same for io, tu, and lui/lei. How do we know which one it is here?

Context and the stated subject! Here, the subject of the second clause is questa impronta (this footprint), which acts as a third-person 'it'. If the subject were missing and the context was ambiguous, you would need to add a pronoun to clarify, like credo che tu sia (I believe that you are).

Does impronta only mean 'footprint'?