yí gè rén wǎnshang chūqù hěn wēixiǎn.

Because 一 changes tone depending on what follows.

In 一个人, the next syllable is 个 (gè), which is fourth tone, so 一 becomes second tone: yí.

A useful rule:

• yī when said alone or for emphasis
• yí before a fourth-tone syllable
• yì before most other tones

So:

• 一个 → yí ge
• 一天 → often yì tiān

Literally, 一个人 means one person. In this sentence, it most naturally means alone / by oneself.

So 一个人晚上出去很危险 is understood as:

• Going out at night alone is dangerous
• It is dangerous for one person to go out at night

This is very common in Mandarin: 一个人 can mean both one person and alone, depending on context.

Because Mandarin usually requires a measure word / classifier between a number and a noun.

So you say:

• 一个人 = one person
• not normally just 一人 in everyday speech

个 is the most common general classifier, and it works naturally with 人 here.

You may also see:

• 一位人 is not correct
• 一位 can be used for one person in a polite way, but usually as 一位老师, 一位客人, etc.
• 一人 does exist, but it sounds more formal, literary, or fixed-expression-like

Because time expressions in Mandarin usually come before the verb.

The basic idea is:

• who / what
  • when
    • verb phrase
      • comment

So:

• 一个人 = a person / alone
• 晚上 = at night
• 出去 = go out
• 很危险 = is dangerous

That makes 晚上出去 = go out at night.

Mandarin usually puts the time first, rather than after the verb as English often can.

It can mean either evening or night, depending on context.

In this sentence, English speakers often translate it as at night, because that sounds more natural with dangerous. But in the evening is not impossible if the context points that way.

So the exact English wording depends on context, but 晚上 covers the general evening/night period.

去 means to go, while 出去 means to go out.

The 出 adds the idea of moving:

• out
• outward
• from inside to outside

So:

• 去 = go
• 出去 = go out

In this sentence, 出去 is more natural because the idea is specifically going out at night, not just going somewhere.

Because the whole phrase 一个人晚上出去 functions as the subject/topic of the sentence.

In English, we often say:

• Going out alone at night is dangerous

Mandarin can do something very similar. The action phrase itself can be what the sentence is about.

So the structure is roughly:

• 一个人晚上出去 = going out alone at night
• 很危险 = is dangerous

Mandarin often leaves out an explicit subject when the sentence is making a general statement.

Not necessarily.

In Mandarin, adjectives often work like verbs, and 很 is commonly placed before them in simple statements. In many cases, 很 does not need to be translated as very.

So:

• 很危险 often just means dangerous or quite dangerous
• not always very dangerous

If you really want strong emphasis, you could say:

• 非常危险 = very dangerous
• 特别危险 = especially dangerous

If you say just 危险 with no 很, it can sound more contrastive or less neutral, as if you are emphasizing dangerous in contrast to something else.

Yes. That is also natural.

Both of these work:

• 一个人晚上出去很危险
• 晚上一个人出去很危险

The difference is mostly about emphasis or flow:

• 一个人晚上出去很危险 starts by highlighting being alone
• 晚上一个人出去很危险 starts by highlighting the time

Mandarin often allows this kind of flexibility with time expressions.

By itself, it sounds like a general statement.

It means something like:

• Going out alone at night is dangerous
• For a person to go out alone at night is dangerous

It is not strongly tied to one specific person unless the context makes that clear.

If you wanted to make it specific, you could say something like:

• 你一个人晚上出去很危险 = It is dangerous for you to go out alone at night

Yes. Mandarin has other ways to express alone, such as:

• 独自
• 自己一个人

For example:

• 独自晚上出去很危险
• 自己一个人晚上出去很危险

But 一个人 is very common, natural, and conversational. It is one of the most basic everyday ways to express the idea of being alone.