Si tu as froid, tu commences à trembler, n'est-ce pas ?

Because French uses avoir with many physical sensations:

• avoir froid = to be cold / to feel cold
• avoir chaud = to be hot
• avoir faim = to be hungry
• avoir soif = to be thirsty

So tu as froid literally looks like you have cold, but it means you are cold or you feel cold.

Tu es froid usually means you are cold in the sense of emotionally cold, or that your body/object is cold as a characteristic, not simply that you feel chilly.

Si means if, so si tu as froid means if you are cold.

That suggests a condition: if this happens, then that happens.

• Si tu as froid, tu commences à trembler. = If you are cold, you start to shiver.
• Quand tu as froid, tu commences à trembler. = When you are cold, you start to shiver.

In many contexts, both can make sense, but the nuance is slightly different:

• si = condition
• quand = when/whenever, something more expected or habitual

French often uses the present + present pattern for general truths, habits, or things that usually happen.

So:

• Si tu as froid, tu commences à trembler.

means something like:

• If you’re cold, you start shivering
• When you get cold, you start shivering

This is not the same as English If you are cold, you will start shivering, even though the meaning can be similar. French does not normally use the future after si in this kind of sentence.

A useful rule:

• after si meaning if, French does not use the future tense in this structure

So Si tu auras froid... would be wrong here.

Because the second clause needs its own subject.

French usually states the subject clearly with each conjugated verb:

• Si tu as froid, tu commences à trembler.

The first tu belongs to as. The second tu belongs to commences.

If you said Si tu as froid, commences à trembler, that would sound like a command:

• If you’re cold, start shivering.

So the repeated tu is necessary because this sentence is making a statement, not giving an order.

Commencer à + infinitive means to begin to / to start to do something.

So:

• commencer à trembler = to start shivering

Examples:

• Il commence à pleuvoir. = It’s starting to rain.
• Je commence à comprendre. = I’m starting to understand.

This is a very common pattern in French:

• commencer à + verb

Because commencer is followed by à before another verb in the infinitive.

Pattern:

• commencer à + infinitive

So:

• tu commences à trembler

This is just how the verb is built in this meaning.

Compare:

• apprendre à faire quelque chose = to learn to do something
• aider à faire quelque chose = to help to do something

You usually have to learn which preposition goes with each verb.

Trembler means to tremble, shake, or shiver.

In this sentence, it means to shiver because of cold.

Frissonner can also mean to shiver, often with a slight nuance of a chill or shudder. In many situations, the two are close, but trembler is the more general word for shaking.

Examples:

• Je tremble de froid. = I’m shivering from cold.
• Je frissonne. = I’m shivering / I have chills.

So here, trembler is perfectly natural.

N'est-ce pas ? is a tag question, like English:

• ..., isn’t it?
• ..., right?
• ..., don’t you?
• ..., aren’t you?

In this sentence it is asking for confirmation:

• Si tu as froid, tu commences à trembler, n'est-ce pas ?
• If you’re cold, you start shivering, right?

It’s a fixed expression and does not change to match the subject the way English tags do. English would say don’t you? here, but French can simply use n’est-ce pas ?

Because n'est-ce pas ? is a fixed tag expression meaning something like isn’t that so?

It does not work like English tag questions, where you change the auxiliary depending on the sentence:

• You’re cold, aren’t you?
• You started, didn’t you?

French often avoids that kind of changing tag and uses the more general n’est-ce pas ?

You may also hear more informal tags such as:

• hein ?
• non ?
• pas vrai ?

But n’est-ce pas ? is standard and clear.

The hyphens are part of the fixed expression n’est-ce pas.

It comes historically from:

• n’ est ce pas which is written as
• n’est-ce pas

French often uses hyphens in set expressions or inversion structures.

So you should learn n’est-ce pas ? as one chunk.

Tu is the singular informal you.
Vous is used for:

• one person in a formal situation
• more than one person

So tu suggests the speaker is talking to:

• a friend
• a child
• a family member
• someone they know well

If you wanted the formal or plural version, you could say:

• Si vous avez froid, vous commencez à trembler, n'est-ce pas ?

Usually, no—not for the normal idea you feel cold.

French strongly prefers:

• tu as froid

If you say:

• tu es froid

it often sounds like:

• your skin/body feels cold to the touch
• you are emotionally cold
• you are cold in manner or attitude

So for the common everyday meaning you’re cold, use avoir froid.

The comma is very common because n’est-ce pas ? is added like a short extra tag at the end:

• ..., n’est-ce pas ?

It helps show a pause, just like in English:

• ..., right?
• ..., isn’t it?

In informal writing, punctuation can vary, but using the comma is standard and a good habit.

Yes. The form can be understood as:

• you start to tremble
• you begin to tremble
• you are starting to tremble

The exact English translation depends on context. In French, the present tense often covers ideas that English may express with either the simple present or the present progressive.

So tu commences à trembler is flexible, but in this sentence it most naturally means a general reaction:

• you start shivering

The given sentence is correct, but in everyday speech people might say simpler or more idiomatic things, for example:

• Si tu as froid, tu te mets à trembler, non ?
• Quand tu as froid, tu trembles, non ?
• Quand t’as froid, tu commences à trembler, non ?

A few notes:

• te mettre à + infinitive also means to start doing something
• non ? is a common casual tag instead of n’est-ce pas ?
• t’as is the spoken contraction of tu as

So the original sentence is perfectly correct, but n’est-ce pas ? can sound a bit more formal or deliberate than everyday conversation.