Learn Croatian: Mladić koji pjeva poznati je pjevač. - The young man who is singing is a famous singer.

In Croatian, tko is mainly used as a question word (like 'Who is that?'). When you want to connect two parts of a sentence and refer back to a noun—like 'the young man who...'—you use the relative pronoun koji. Because it refers back to mladić (a masculine singular noun) and acts as the subject of the singing action, it stays in the nominative masculine form koji.

Why is the verb je placed right in the middle of poznati pjevač?

je is an enclitic, which is a 'weak' word that usually wants to be in the second position of a sentence or clause. Here, the whole subject phrase (Mladić koji pjeva) acts as the first block. You could say Mladić koji pjeva je poznati pjevač, but placing the enclitic after the first word of the next phrase (poznati) is also very natural in Croatian and breaks up the sentence smoothly.

Does the -i at the end of poznati mean it is plural?

No, poznati is singular here, just like pjevač. The -i ending is the 'definite' form of the masculine adjective. While masculine adjectives often end in a consonant (like poznat), they take an -i when referring to a specific, already-identified noun. Since we have already identified the young man using the koji clause, the definite poznati sounds most natural.