Teatrul de amatori pregătește premiera.

Because Romanian usually puts the definite article at the end of the noun.

• teatru = theater / a theater
• teatrul = the theater

So -ul is the masculine/neuter singular definite article here. Romanian teatru is a neuter noun, and in the singular neuter nouns use masculine-type forms, so teatrul means the theater.

In Romanian, it is very common to use noun + de + noun to describe a type or category.

So:

• teatru de amatori = literally theater of amateurs
• in natural English, this is usually amateur theater

Romanian can sometimes use adjectives where English does, but in this case de amatori is the normal and idiomatic way to express the idea.

Amatori is the plural form of amator, meaning amateur.

So:

• amator = amateur
• amatori = amateurs

In de amatori, the plural is used in a general sense: a theater made up of amateurs / an amateur theater.

Because de amatori is being used as a general descriptive phrase, not to mean of the amateurs in a specific sense.

Compare:

• teatru de amatori = amateur theater
• teatrul amatorilor = the theater of the amateurs

The sentence uses de amatori because it describes what kind of theater it is.

Because the subject teatrul de amatori is third person singular.

The verb a pregăti means to prepare. In the present tense:

• eu pregătesc = I prepare
• tu pregătești = you prepare
• el/ea pregătește = he/she/it prepares

Since teatrul is singular, Romanian uses pregătește.

Yes. It is the present tense, and in Romanian the present tense often covers both:

• prepares
• is preparing

So Teatrul de amatori pregătește premiera can mean either The amateur theater prepares the premiere or, more naturally in English, The amateur theater is preparing the premiere, depending on context.

Because premiera is the definite form.

• premieră = a premiere
• premiera = the premiere

For many feminine nouns ending in -ă, the definite singular form ends in -a:

• casă → casa
• premieră → premiera

So here the sentence is talking about a specific premiere: the premiere.

Yes, it is the direct object, so it is functioning as an accusative.

However, in Romanian, many nouns have the same form for nominative and accusative. That is what happens here.

So:

• nominative: premiera
• accusative: premiera

The role is understood from the sentence structure, not from a different noun ending.

The normal order here is Subject + Verb + Object:

• Teatrul de amatori = subject
• pregătește = verb
• premiera = object

That is the most neutral order.

Romanian word order is somewhat flexible, though, especially for emphasis. For example, you could move things around in other contexts, but Teatrul de amatori pregătește premiera is the straightforward, standard version.

Yes, it matters because gender affects articles and some other forms.

• teatru is neuter
• amator is masculine in its basic form
• premieră is feminine

This is why you get:

• teatrul with -ul for the definite singular
• premiera with -a for the definite singular feminine

Romanian gender is important because it influences how nouns and related words are formed.

The most important special letters here are:

• ă = a short central vowel, like a very reduced uh sound
• ș = sh

So pregătește is pronounced roughly like preh-guh-TESH-teh.

A rough English-style pronunciation of the whole sentence is:

TEA-trul de a-ma-TOR-ee pre-guh-TESH-te pre-mee-E-ra

This is only approximate, but it helps you notice the special sounds, especially ă and ș.