eigakan ni hairu mae ni, sumaho wo muon ni simasu.

Because 入る is an intransitive verb in Japanese.

With 入る, the place you go into is marked with に:

• 部屋に入る = enter the room / go into the room
• 映画館に入る = enter the movie theater

This is different from English, where enter takes a direct object. In Japanese, 入る works more like go into.

If you want a transitive verb meaning put something in, that is 入れる:

• かばんに本を入れる = put a book in the bag

Because when a verb comes before a noun like 前, Japanese normally uses the plain form.

So:

• 入る前に = before entering
• 食べる前に = before eating
• 寝る前に = before sleeping

Here, 前 is a noun, and 入る is modifying it.

Also, this does not make the whole sentence casual. The main verb is します, which is polite, so the sentence is still polite overall. It is very normal in Japanese for a sentence to be polite at the end, while verbs inside subordinate parts stay in plain form.

Verb + 前に means before doing that verb.

So:

• 映画館に入る前に = before entering the movie theater

The に after 前 marks the time point at which the main action happens.

A useful comparison:

• 映画館の前 = in front of the movie theater
• 映画館に入る前に = before entering the movie theater

So in this sentence, 前 is about time, not location.

They are doing two different jobs.

1. The first に in 映画館に入る

   • marks the destination of 入る
   • 映画館に入る = enter the movie theater

2. The second に in 前に

   • marks the time expression
   • 入る前に = before entering

So even though they look the same, they are not redundant.

This uses the pattern:

X を Y にする

which means:

make X into Y
or
set X to Y

So here:

• スマホ = smartphone
• を = marks the thing being changed
• 無音にする = make silent / set to silent

So スマホを無音にします means set the phone to silent.

You can see the same pattern in other sentences:

• 部屋をきれいにする = make the room clean
• 音を小さくする = make the sound quieter
• 設定をオンにする = turn the setting on

無音 literally means no sound or silence.

In this sentence, it is being used as the target state in 無音にする, so the meaning is make it silent or set it to silent.

You can think of it as functioning a bit like a noun or na-adjective-like word in this pattern:

• 静かにする = make it quiet
• 同じにする = make it the same
• 無音にする = make it silent

One note: for an actual phone, more common everyday expressions are often:

• マナーモードにします = put it on manner mode
• サイレントモードにします = put it on silent mode
• 消音にします = mute it

But grammatically, 無音にします is perfectly understandable.

Because スマホ is the direct object in the pattern X を Y にする.

So:

• スマホを無音にします = set the smartphone to silent

If you used は, the nuance would change. It would make スマホ the topic, often with some contrast:

• スマホは無音にします = as for the phone, I set it to silent

That is possible, but the neutral grammar for make/set X into Y is X を Y にする.

The subject is omitted because Japanese often leaves out information that is obvious from context.

Depending on the situation, this could mean:

• I set my phone to silent
• we set our phones to silent
• you should set your phone to silent

Japanese also often omits possessives when they are obvious, so スマホ can naturally mean my phone, your phone, or one’s phone, depending on context.

No. Even though 前 can mean front, here it means before in time.

Compare these:

• 映画館の前にいます = I am in front of the movie theater
• 映画館に入る前に、スマホを無音にします = Before entering the movie theater, I set my phone to silent

The key difference is:

• noun + の前 = in front of something
• verb + 前に = before doing something

So this sentence is about timing, not physical position.