Завтра мы собираемся играть в теннис на стадионе.

In this sentence, собираться + infinitive is a very common pattern meaning to be going to do something or to intend to do something.

So:

• мы собираемся играть = we are going to play / we plan to play

By itself, собираться can also mean to get together, to get ready, or to gather oneself, depending on context. But when it is followed by an infinitive, it usually means to be planning/intending to do that action.

So here, you should understand it as a future plan, not as we are physically gathering to play.

Because after собираться, Russian normally uses the infinitive of the main action.

• мы собираемся играть = we are going to play
• not мы собираемся играем

This works much like English to be going to + verb:

• We are going to play
• not We are going to we play

So:

• собираемся = the conjugated verb
• играть = the action being planned

The -ся ending marks the verb as reflexive. In many Russian verbs, this ending changes the meaning or creates a standard dictionary form that does not always translate literally as an English reflexive.

Here:

• собирать = to collect, gather something
• собираться = to get ready, gather, plan, intend

So собираться is just the normal verb used in the expression собираться + infinitive.

Also note the full form here:

• мы собираемся
• the ending -емся shows 1st person plural: we

Теннис is in the accusative case, but for this noun the accusative looks the same as the nominative:

• nominative: теннис
• accusative: теннис

After играть в when talking about a game or sport, Russian uses the accusative:

• играть в теннис
• играть в футбол
• играть в шахматы

So the pattern is:

• играть в + game/sport

This is just how Russian expresses to play tennis, to play football, etc.

Russian often uses на with places that English may think of as in. A stadium is one of those places.

So:

• на стадионе = at the stadium

This is the normal Russian choice.

The noun стадион here is in the prepositional case because it follows на in a location meaning:

• стадион → на стадионе

Compare:

• на стадионе = at the stadium
• на улице = on the street / outside
• на работе = at work

You should usually learn these as fixed combinations: на стадионе is simply the standard expression.

Стадионе is the prepositional case singular of стадион.

Base form:

• стадион

After на when it means in/at a place (location, not movement), Russian uses the prepositional case:

• на стадионе

So:

• играть на стадионе = to play at the stadium

If there were movement to the stadium, Russian would normally use a different case:

• на стадион = to the stadium

So the contrast is:

• на стадионе = at the stadium
• на стадион = to the stadium

In present-tense Russian, the verb to be is usually omitted.

English says:

• We are going to play

Russian says:

• Мы собираемся играть

There is no separate present-tense word for are here. The meaning is already built into the verb собираемся, which means we are planning / we are going to.

This is very normal in Russian:

• Я студент. = I am a student.
• Мы дома. = We are at home.

So do not expect a separate word for am/is/are in most present-tense Russian sentences.

Yes, the word order can change.

Russian word order is more flexible than English because case endings show grammatical relationships. Putting Завтра first gives the time expression extra prominence:

• Завтра мы собираемся играть в теннис на стадионе. = Tomorrow, we’re going to play tennis at the stadium.

Other possible orders include:

• Мы завтра собираемся играть в теннис на стадионе.
• Мы собираемся завтра играть в теннис на стадионе.

These all mean roughly the same thing, but the emphasis shifts slightly.

Putting завтра first is very natural when you want to frame the whole sentence with tomorrow.

Yes, мы собираемся играть usually suggests an intention, plan, or arrangement.

• мы собираемся играть = we’re going to play / we’re planning to play

By contrast:

• мы будем играть = we will be playing / we will play

Будем играть is a more neutral future statement.
Собираемся играть stresses that this is something already intended or planned.

So the difference is often:

• собираемся играть → planned intention
• будем играть → future action, less focus on the plan itself

Because the subject is мы = we, so the verb must match 1st person plural.

The verb собираться in the present tense goes like this:

• я собираюсь = I am going to
• ты собираешься = you are going to
• он / она собирается = he/she is going to
• мы собираемся = we are going to
• вы собираетесь = you are going to
• они собираются = they are going to

So мы собираемся is exactly the form that matches we.

Yes, a few useful ones:

• завтра → stress on the first syllable: ЗА-втра
• собираемся → stress on -РА-: со-би-РА-ем-ся
• играть → stress on the second syllable: и-ГРАТЬ
• теннис → stress on the first syllable: ТЕннис
• стадионе → stress on -О-: ста-ди-О-не

A rough pronunciation guide:

• Завтра мы собираемся играть в теннис на стадионе
• ZAF-tra my sa-bee-RA-yem-sya ee-GRAT' f TEN-nees na sta-dee-O-nye

Also note that Russian в before another consonant can sound weak, so in fast speech в теннис may sound close to ф теннис.

Russian has no articles like a or the.

So:

• на стадионе can mean at the stadium or sometimes just at a stadium, depending on context.

Usually the context tells you whether the meaning is definite or indefinite. In this sentence, English naturally uses the stadium, but Russian does not need a separate word for that.

This is normal for all Russian nouns:

• я вижу машину = I see a car / the car
• мы на стадионе = we are at the stadium

Context does the job that articles do in English.