Nous sommes obligés d’acheter encore du brocoli, parce que celui d’hier n’a pas suffi.

Both are possible, but they are not exactly the same in tone.

• être obligé de + infinitive = to be forced/required to, to have to
• devoir + infinitive = must / have to

So:

• Nous sommes obligés d’acheter... suggests a situation where the speakers feel compelled by circumstances.
• Nous devons acheter... is a bit more neutral and can sound like a simple obligation or necessity.

In everyday French, both can work here. Être obligé de often feels slightly stronger or more descriptive.

Because obligés is an adjective, and it agrees with nous.

Here, nous sommes obligés literally means we are obligated.

Agreement rules:

• masculine singular: obligé
• feminine singular: obligée
• masculine plural: obligés
• feminine plural: obligées

So:

• Nous sommes obligés = used for a group of men, or a mixed group, or when gender is unspecified
• Nous sommes obligées = used if the whole group is female

Because the expression is être obligé de + infinitive.

So the full structure is:

• être obligé de faire quelque chose
• être obligé d’acheter = to be obliged to buy

The de becomes d’ before a vowel sound, which is called elision:

• de acheter → d’acheter

Here encore means again or some more.

So:

• acheter encore du brocoli = buy more broccoli / buy broccoli again

In this sentence, the natural English idea is buy more broccoli, because the broccoli from yesterday was not enough.

Encore can mean different things depending on context:

• again
• still
• more

Here, more / again is the right sense.

Because French usually uses a partitive article for an unspecified quantity of something uncountable or treated as a mass noun.

• du brocoli = some broccoli

Compare:

• Je mange du pain. = I eat bread / some bread
• On a acheté du fromage. = We bought some cheese

Why not the others?

• le brocoli would mean broccoli in general or the broccoli
• brocoli without an article is usually not correct in normal French here

So acheter du brocoli is the normal way to say buy some broccoli.

Celui means the one.

So:

• celui d’hier = the one from yesterday

It replaces a masculine singular noun that has already been mentioned. Here, it stands for le brocoli.

So the sentence avoids repeating brocoli:

• ...parce que celui d’hier n’a pas suffi
• literally: because the one from yesterday was not enough

French often uses demonstrative pronouns like this:

• celui = masculine singular
• celle = feminine singular
• ceux = masculine plural
• celles = feminine plural

Because de becomes d’ before a vowel sound.

• de + hier → d’hier

This is very common in French:

• de école → d’école
• de ami → d’ami
• de habitude → d’habitude

So celui d’hier literally means the one of yesterday, which in natural English becomes the one from yesterday.

It is the passé composé, a very common past tense in French.

The verb is suffire = to be enough / to suffice.

Passé composé structure:

• auxiliary verb (avoir here) + past participle

So:

• a suffi = was enough / sufficed
• n’a pas suffi = was not enough / did not suffice

The pieces are:

• n’... pas = negation
• a = has
• suffi = past participle of suffire

So celui d’hier n’a pas suffi means yesterday’s one wasn’t enough.

Because French often uses the verb suffire, which already contains the idea of being enough.

So instead of saying something like did not be enough, French simply says:

• n’a pas suffi = was not enough

Examples:

• Ça suffit. = That’s enough.
• Ce n’est pas suffisant. = It’s not sufficient.
• Le pain n’a pas suffi. = The bread wasn’t enough.

So in your sentence, suffire is doing the work that English expresses with be enough.

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

In simple terms:

• Il a suffi. = It was enough.
• Il n’a pas suffi. = It was not enough.

With compound tenses like the passé composé, ne ... pas goes around the auxiliary verb:

• a is the auxiliary
• so you get n’a pas

This pattern is very important:

• Je n’ai pas mangé.
• Nous n’avons pas vu.
• Elle n’est pas venue.

Yes. French has several natural ways to express the same basic idea.

For example:

• Nous devons encore acheter du brocoli...
• Il faut qu’on achète encore du brocoli...
• On est obligés d’acheter encore du brocoli...

These all mean roughly the same thing, but with slightly different style or emphasis:

• nous sommes obligés de = we are obliged / forced to
• nous devons = we have to / must
• il faut que... = it’s necessary that...
• on est obligés de = more conversational than nous sommes obligés de

Your original sentence is completely natural and grammatically correct.