Learn Hardcore French: Fais attention, autrement tu vas tomber. - Be careful, otherwise you are going to fall.

Because this is the imperative (a command). The imperative forms of faire are: fais (tu), faisons (nous), faites (vous). Fait is the 3rd person singular of the present indicative (e.g., il fait), not a command.

Is Fais attention a fixed expression? Why is there no article before attention?

Yes. Faire attention is a set phrase meaning “to be careful / to pay attention,” and attention takes no article in this idiom. Examples: Fais attention à la marche. / Il fait attention en conduisant.

How would I say this politely or to several people?

Use the vous-form: Faites attention, autrement vous allez tomber. You can also keep the singular warning but be polite with vous: Faites attention !

Could I say Sois prudent(e) instead of Fais attention?

Yes. Sois prudent(e) means “be careful/prudent” and sounds a bit more general about one’s behavior, whereas Fais attention often targets an immediate risk. Both are fine in a warning.

What’s the difference between autrement and sinon here?

Both mean “otherwise” in this context and are interchangeable: Fais attention, sinon tu vas tomber. Register nuance: sinon is very common and colloquial; autrement is a bit more formal. Note that sinon can also mean “other than that” (e.g., Sinon, tout va bien.).

Why is it tu vas tomber instead of tu tomberas?

Tu vas tomber is the near future (aller + infinitive), which feels immediate and is the natural choice for a warning. Tu tomberas (simple future) sounds more like a neutral prediction and is less typical in an urgent warning.

Can I leave out autrement?

Yes: Fais attention, tu vas tomber. The meaning is clear from context. You can also use sinon instead of autrement.

Is tomber reflexive? Should it be tu vas te tomber?

No. Tomber is intransitive and not reflexive in this sense: tu vas tomber is correct. Reflexive forms occur only in special idioms (e.g., ils se sont tombé dessus “they fell on each other”).

How do I say what to be careful of?

Use faire attention à + noun/pronoun/infinitive:

Fais attention à la marche.
Fais attention à lui/à eux. (for people; don’t use y with people)
Fais attention à ça. / Fais-y attention. (for things)
Fais attention à ne pas tomber. (with an infinitive)

Pronunciation tips for the whole sentence?

Typical pronunciation: [fɛ(z)‿atɑ̃sjɔ̃ | otʁəmɑ̃ | ty va tɔ̃be]. The liaison in fais‿attention ([z] sound) is optional but common. Autrement is [otʁəmɑ̃] (final -t silent), tomber is [tɔ̃be].

Is the comma with autrement necessary? Could I use other punctuation?

A comma is standard to set off the “otherwise” clause: Fais attention, autrement… A semicolon also works in careful writing: Fais attention ; autrement, tu vas tomber. In French, an (optional) thin space precedes an exclamation mark: Fais attention !

What are some common mistakes to avoid?

Writing Fait attention (wrong) instead of Fais attention (right).
Writing faîtes (wrong) instead of faites (right).
Saying tu va tomber (wrong) instead of tu vas tomber.
Using a conjugated verb after aller: it must be the infinitive (vous allez tomber, not vous allez tombez).
Dropping the final -r in writing: tu vas tomber, not tu vas tombe.
Adding redundant en bas (France): tomber already implies “down” (though tomber en bas may be heard regionally, e.g., in Québec).

Is autrement always “otherwise”? Can it mean something else?

It can also mean “differently”: Fais-le autrement = “Do it differently.” In your sentence it means “otherwise/if not.”

Can I start a sentence with Autrement?

Yes, if it links back to the previous idea: Autrement, tu vas tomber. On its own without prior context, it sounds abrupt.

Any useful synonyms for the verbs here?

For the warning: Fais gaffe (colloquial), Prends garde (formal/literary), Sois prudent(e).
For tomber depending on nuance: trébucher (to trip), glisser (to slip), chuter (to fall, more formal/sport). Example: Fais attention, sinon tu vas glisser.