xiōngdì yìqǐ qù chēguǎnsuǒ.

In this sentence, 兄弟 most naturally means brothers.

Literally, 兄 is older brother and 弟 is younger brother, but together 兄弟 is a common word meaning brothers as a group.

Depending on context, 兄弟 can also mean bro / buddy / close male friend, but if there is no special context, learners should usually read it literally as brothers.

Mandarin often does not mark plurals when the meaning is already clear from context.

So 兄弟 can already mean brothers without needing 兄弟们.

兄弟们 is possible, but it often sounds more like:

• addressing a group: Brothers! / Guys!
• emphasizing the group strongly

In a simple statement, 兄弟一起去车管所。 is perfectly natural.

Yes. 兄弟 is the subject.

The basic structure is:

兄弟 + 一起 + 去 + 车管所
subject + adverb + verb + destination

So the brothers are the ones doing the action of going.

Because 一起 works like an adverb here, meaning together.

In Mandarin, adverbs usually come before the main verb. So:

• 一起去 = go together

This is the normal pattern.

A good rule to remember is:

subject + adverb + verb + object/place

So 兄弟一起去车管所 follows standard Mandarin word order.

Because in Mandarin, 去 + place already means go to + place.

So:

• 去车管所 = go to the vehicle administration office

You do not need a separate word matching English to in this kind of sentence.

Mandarin often expresses movement like this:

• 去学校 = go to school
• 去北京 = go to Beijing
• 去车管所 = go to the DMV / vehicle administration office

车管所 is short for 车辆管理所.

It refers to the government office that handles vehicle-related administration, such as:

• vehicle registration
• license plates
• driver-related paperwork
• other car administration matters

In English, learners often think of it as something like the DMV, though the exact government system may differ by country.

Because 是 is not used with ordinary action verbs like 去.

是 is mainly used to connect one noun phrase to another, like:

• 他是老师。 = He is a teacher.

But here the sentence is about an action:

• 去 = go

So you just use the verb directly:

• 兄弟一起去车管所。

Not:

• 兄弟是一起去车管所。 in normal usage

Because this sentence is just a plain statement about the action, without specifically marking completion or change of situation.

Adding 了 would change the feel:

• 兄弟一起去车管所。
  A neutral statement: The brothers go / are going together to the DMV.

• 兄弟一起去车管所了。
  This suggests they went / have gone off to the DMV, or that this is now the situation.

So the version without 了 is simpler and more neutral.

This is because of the tone-change rule for 一.

一 is written with first tone in dictionaries: yī.
But in actual speech, its tone often changes depending on the next syllable.

Before a third-tone syllable like 起 (qǐ), 一 becomes fourth tone:

• 一起 → yìqǐ

So even though the dictionary form is yī, the correct pronunciation here is yìqǐ.

No, not normally.

兄弟 specifically refers to brothers or sometimes male close friends.

If you want to say siblings in general, including sisters, Mandarin usually uses:

• 兄弟姐妹 = brothers and sisters
• 手足 in more formal/literary language

So 兄弟 by itself does not usually mean all siblings.

It is similar, but not exactly the same.

English:

• The brothers go together to the DMV.

Mandarin:

• 兄弟一起去车管所。

A helpful Mandarin pattern is:

subject + manner/adverb + verb + destination

Here:

• 兄弟 = subject
• 一起 = together
• 去 = go
• 车管所 = destination

So the sentence order is very natural for Mandarin.