Andrei repetă sunetul încet, iar Ana îl ajută cu pronunția.

Because -ul is the Romanian definite article attached to the end of the noun.

• sunet = a sound
• sunetul = the sound

Romanian usually puts the at the end of the word instead of using a separate word like English does. Also, sunet is a neuter noun, and in the singular neuter nouns behave like masculine nouns, which is why the definite ending is -ul.

Here încet is an adverb modifying the verb repetă. In this sentence it means slowly or carefully.

So:

• repetă sunetul încet = repeats the sound slowly

In other contexts, încet can also mean quietly, so learners often notice that it has more than one possible translation.

îl is a direct object pronoun meaning him or it. Here it refers to Andrei.

So:

• Ana îl ajută = Ana helps him

Romanian very often uses these short object pronouns before the verb.

Because unstressed object pronouns like îl normally come before the conjugated verb in Romanian.

So the normal order is:

• îl ajută = helps him

and not:

• ajută îl

That second version sounds ungrammatical in standard Romanian.

iar usually means something like and, while, or whereas, depending on context.

In this sentence, it connects two clauses and signals a small shift of focus:

• Andrei is doing one action
• Ana is doing another

So iar feels natural because the subject changes. și is the more basic word for and, but iar often sounds better when contrasting or balancing two parts of a sentence.

Because Romanian uses the pattern a ajuta pe cineva cu ceva, which means to help someone with something.

So:

• îl ajută cu pronunția = she helps him with the pronunciation

The preposition cu simply means with here.

Because pronunția is the definite form.

• pronunție = pronunciation
• pronunția = the pronunciation

This noun is feminine, so the definite article is added as -a:

• pronunție → pronunția

Romanian attaches the definite article to the end of feminine singular nouns too.

Both are 3rd person singular present tense verb forms.

• Andrei repetă = Andrei repeats
• Ana ajută = Ana helps

Romanian verb endings show person and number, so you do not need an extra subject pronoun like he or she when the subject is already clear.

In standard Romanian, with a person as a direct object, you normally use pe, and very often you also use the matching object pronoun.

So the natural full version is:

• Ana îl ajută pe Andrei

Here:

• pe Andrei marks the person as the direct object
• îl repeats that object in pronoun form

In your original sentence, pe Andrei is simply omitted because îl already tells us that Ana is helping him.

A few useful ones here are:

• ă as in repetă, ajută: a short central vowel, somewhat like the a in about
• î as in încet, îl: a close central vowel that does not exist exactly in English
• ț as in pronunția: pronounced ts, like the end of cats

So:

• încet begins with the Romanian î
• pronunția includes ț = ts

These letters are important because they change pronunciation and meaning, so they should not be ignored.