Learn Hardcore French: Hier, le vent soufflait plus fort. - Yesterday, the wind was blowing harder.

The imparfait expresses background actions, ongoing states or habitual events in the past. Here, “le vent soufflait” sets a scene or describes how things were over a period of time yesterday.
Using passé composé (e.g. “le vent a soufflé”) would present it as a completed, punctual event—“the wind blew” as a single occurrence—rather than an ongoing situation.

How do you form the imparfait of souffler?

Take the nous form in the present: nous soufflons.
Remove -ons to get the stem: souffl-.
Add the imparfait endings: -ais, -ais, -ait, -ions, -iez, -aient.
• For il/elle, you add -ait, giving soufflait.

Why is it fort and not forte (agreeing with vent, which is masculine anyway, or treating “fort” as adjective)?

Here fort is an adverb modifying the verb soufflait (“was blowing strongly/hard”). Adverbs in French are invariable—no agreement in gender or number. If you treated fort as an adjective (describing “the wind”), you’d need fort (masculine), but then you’d say le vent était fort, not soufflait fort.

What does plus fort mean here, and why isn’t there a que (“than …”) in the sentence?

Plus fort is a comparative meaning “more strongly” or “harder.” In French you can leave the second term of comparison unstated when it’s clear from context (e.g. “yesterday it was stronger than today”). This is called an elliptical comparative. If you wanted to be explicit you could say:
• Hier, le vent soufflait plus fort que maintenant.

Could you say Hier, le vent soufflait plus fortement instead?

Yes. Fortement is the regular adverb derived from fort. Both are correct:
• soufflait plus fort (more colloquial, very common)
• soufflait plus fortement (a bit more formal or literary)
Usage choice is mostly stylistic.

Can you intensify it even more by adding beaucoup?

Absolutely. You can say:
• Hier, le vent soufflait beaucoup plus fort.
This simply strengthens the comparative: “yesterday the wind was blowing a lot harder.”

Why is there a definite article le before vent? Could you say just Vent soufflait…?

In French, natural phenomena and weather elements often take the definite article when speaking generally:
• Le vent, le soleil, la neige…
Saying just Vent soufflait would be ungrammatical—le is required to identify “the wind” as the subject.

Where does hier usually go in the sentence? Can you move it?

Hier is an adverb of time and can appear:
• At the very beginning (as here) for emphasis: Hier, le vent soufflait…
• Or after the verb: Le vent soufflait hier…
Both are correct; placing it first simply highlights “yesterday” as the time frame.

Could you instead say Hier, le vent était plus fort? What’s the nuance?

Yes, you can.
• Soufflait plus fort focuses on the action—how strongly the wind was blowing.
• Était plus fort describes the wind’s strength as a state or quality.
The first is more dynamic (action), the second more static (attribute).