Ne mange pas tout le gâteau d'un seul coup.

That is the standard way to make a verb negative in French.

• mange = eat
• ne mange pas = don’t eat

In this sentence, it is a negative command, so ne comes before the verb and pas comes after it:

• Ne mange pas ... = Don’t eat ...

This is the normal pattern for negative imperatives in French.

Because this is the tu form of the imperative of an -er verb.

The verb is manger. In the present tense, you normally say:

• tu manges

But in the affirmative tu imperative, French usually drops the final -s with regular -er verbs:

• Mange ! = Eat!

The same verb form is used here inside the negative imperative:

• Ne mange pas ! = Don’t eat!

So even though the hidden subject is tu, the form is mange, not manges.

It is a command to tu — one person in an informal context.

So this sentence is something you would say to:

• a child
• a friend
• a family member
• someone you address with tu

If you wanted to say it to vous instead, you would say:

• Ne mangez pas tout le gâteau d’un seul coup.

If you wanted to say it to nous, you would say:

• Ne mangeons pas tout le gâteau d’un seul coup.

In the imperative, French normally leaves out the subject pronoun.

So instead of saying:

• Tu ne manges pas ...

you say:

• Ne mange pas ...

That is normal for commands and instructions. English does the same thing:

• You don’t eat all the cake at once = statement
• Don’t eat all the cake at once = command

Because gâteau is singular, and tout here means the whole or all of the.

• tout le gâteau = the whole cake / all the cake
• tous les gâteaux = all the cakes

So:

• tout + singular noun = the whole ...
• tous/toutes + plural noun = all the ...

Since gâteau is singular, tout le gâteau is correct.

It means the whole cake or all of the cake.

French often uses tout le / toute la / tous les / toutes les to express this idea:

• tout le pain = all the bread / the whole loaf
• toute la journée = the whole day
• tous les livres = all the books

So Ne mange pas tout le gâteau means Don’t eat the whole cake.

Yes. Gâteau is a masculine noun, so it takes le in the singular:

• le gâteau = the cake

If it were feminine, you would expect la instead.

For example:

• la tarte = the tart/pie

So tout le gâteau is correct because gâteau is masculine singular.

D’un seul coup is a very common expression meaning:

• all at once
• in one go
• in one shot

Literally, it is something like in a single blow/stroke, but in everyday French it usually means doing something all at once rather than gradually.

So in this sentence, it adds the idea:

• Don’t eat the whole cake all at once.

It is an idiomatic expression, so it is best learned as a chunk: d’un seul coup.

Because de contracts before a vowel sound.

• de + un becomes d’un

This happens very often in French for smoother pronunciation:

• d’un côté
• d’accord
• d’habitude

So d’un seul coup is just the normal shortened form of de un seul coup.

Because in French, pas normally stays close to the verb.

So the structure is:

• Ne + verb + pas + rest of sentence

That gives:

• Ne mange pas tout le gâteau ...

If you moved things around, the meaning could change or sound unnatural.

For example, compare:

• Ne mange pas tout le gâteau. = Don’t eat the whole cake.
• Ne mange pas tout. = Don’t eat everything.

So pas is negating the action mange, not sitting directly next to the noun the way English might suggest.

Yes, in informal spoken French, many speakers drop ne:

• Mange pas tout le gâteau d’un seul coup.

That is very common in conversation. However, in standard written French and in careful speech, you should keep ne:

• Ne mange pas tout le gâteau d’un seul coup.

So for learners, the full form with ne is the safest one to use.

It actually can. En une fois also means in one go / in one sitting, and in many contexts it would work.

For example:

• Ne mange pas tout le gâteau en une fois.

That sounds natural too.

But d’un seul coup is a very common idiomatic expression and often feels a bit more vivid or emphatic. Both are possible, but d’un seul coup is an expression you will hear a lot.

You would use the vous imperative:

• Ne mangez pas tout le gâteau d’un seul coup.

This can mean either:

• Don’t eat the whole cake all at once — said politely to one person
• Don’t eat the whole cake all at once — said to more than one person

So the original Ne mange pas ... is informal singular, while Ne mangez pas ... is polite singular or plural.

No. Very often it is not literal at all.

It can be used for many actions to mean all at once, even when there is no physical blow or stroke involved:

• Il a tout dépensé d’un seul coup. = He spent it all at once.
• Elle a compris d’un seul coup. = She understood all at once.

So in your sentence, it simply means all at once, not in one literal bite or one literal movement.