Бабушка сказала, что дрожжи должны быть свежими, иначе тесто не поднимется.

Because бабушка is a feminine noun, so the past-tense verb must agree with it in gender.

• сказать = to say
• сказал = he said
• сказала = she said
• сказало = it said
• сказали = they said

So Бабушка сказала means Grandma said.

Что here means that and introduces a subordinate clause.

So the structure is:

• Бабушка сказала = Grandma said
• что дрожжи должны быть свежими = that the yeast must be fresh

In English, that is often optional: Grandma said (that) the yeast must be fresh. In Russian, что is normally used in this kind of sentence.

Yes, дрожжи is normally a plural-only noun in Russian. This is one of those nouns that refers to one substance or concept in English but behaves grammatically as plural in Russian.

So Russian treats дрожжи a bit like:

• ножницы = scissors
• часы = clock/watch
• деньги = money

That is why the sentence has plural agreement:

• дрожжи должны...
• not дрожжи должно...

Even though English says yeast is, Russian says, grammatically, yeast are.

Because Russian normally uses быть with this kind of meaning: must be.

• должны = must / are supposed to
• быть = be
• свежими = fresh

So:

• дрожжи должны быть свежими = the yeast must be fresh

Without быть, the sentence would sound incomplete or unnatural here. Russian often keeps быть in constructions with должен when describing a required state or quality.

Because after быть in this kind of sentence, Russian very often uses the instrumental case for the predicate adjective.

Here:

• дрожжи = plural
• свежими = instrumental plural of свежий

So:

• быть свежими = to be fresh

This is a very common pattern:

• Он был уставшим = He was tired
• Она хочет быть врачом = She wants to be a doctor
• Они должны быть готовыми = They must be ready

You may sometimes see nominative adjectives in other contexts, but быть + instrumental is very standard and natural here.

It can overlap with all of those in English, depending on context. Here, должны means something like:

• must be
• need to be
• are supposed to be

In this sentence, the sense is practical rather than moral:

• The yeast has to be fresh, otherwise the dough won’t rise.

So it is not about obligation in the sense of rules; it is about a necessary condition.

Иначе means otherwise.

The sentence is structured as:

• Бабушка сказала, что дрожжи должны быть свежими,
• иначе тесто не поднимется.

So the second part gives the consequence if the condition is not met:

• otherwise, the dough won’t rise

The comma is used because иначе introduces a separate clause with its own idea.

Поднимется comes from the verb подняться, which is a perfective verb meaning to rise, to go up, or in the case of dough, to rise / leaven properly.

So:

• тесто не поднимется = the dough will not rise

Why future? Because perfective verbs form a simple future:

• поднимется = it will rise

The sentence is talking about a future result: if the yeast is not fresh, the dough will not rise.

Russian often omits pronouns when they are unnecessary. In this sentence, the second clause switches to a new subject anyway:

• first subject: дрожжи
• second subject: тесто

So there is no need to repeat anything like they. Russian is generally more comfortable than English with leaving out pronouns when the meaning is already clear from context.

Because they are simply different nouns with different grammatical behavior.

• тесто = dough, a normal singular neuter noun
• дрожжи = yeast, a plural-only noun

So the verb forms reflect that:

• дрожжи должны = yeast must
• тесто не поднимется = the dough will not rise

This mismatch can feel strange to English speakers, but it is just something to memorize with the noun.

The given word order is natural and neutral:

• Бабушка сказала, что дрожжи должны быть свежими, иначе тесто не поднимется.

Russian word order is more flexible than English, so some parts could move for emphasis. For example, you might hear:

• Бабушка сказала, что иначе тесто не поднимется, если дрожжи не свежие.
• Дрожжи должны быть свежими, сказала бабушка, иначе тесто не поднимется.

But these versions change the focus or rhythm a bit. For a learner, the original sentence is the best neutral model.

Сказала is from сказать, a perfective verb meaning said as a completed act: she said it once, as a specific statement.

• Бабушка сказала... = Grandma said...

If you used говорила instead, it would come from говорить and would usually suggest something more ongoing, repeated, or less focused on one completed statement:

• Бабушка говорила, что... = Grandma used to say / was saying that...

So сказала is the natural choice when reporting one specific thing she said.

Not in this context. With дрожжи, свежие clearly means fresh in the food/baking sense: still active, not old or ineffective.

Russian свежий can have several meanings depending on context, for example:

• свежий хлеб = fresh bread
• свежий воздух = fresh air
• свежие новости = fresh/latest news

But with дрожжи, the intended meaning is definitely fresh yeast.