Подъём теста оказался медленным.

Because подъём normally takes the genitive to show what is rising or being lifted: подъём чего? — теста.

So the structure is:

• подъём = rise / rising / lifting
• теста = of the dough / of the test

This is very similar to English the rise of the dough, where English uses of and Russian uses the genitive case.

After оказаться in the sense of to turn out to be, Russian usually puts the predicate noun or adjective in the instrumental case.

So:

• медленный = nominative masculine singular
• медленным = instrumental masculine/neuter singular

That is why you get:

• Подъём теста оказался медленным.

This is a very common pattern:

• Он оказался правым. = He turned out to be right.
• Урок оказался трудным. = The lesson turned out to be difficult.

Оказался means turned out to be or proved to be.

It is not just a plain was. It suggests that the speaker learned the result after some expectation, observation, or uncertainty.

Compare:

• Подъём теста был медленным. = The rise of the dough was slow.
  
  • a more neutral statement
• Подъём теста оказался медленным. = The rise of the dough turned out to be slow.
  
  • suggests a discovered result

So оказался adds the idea of it turned out that...

Because it agrees with подъём, which is a masculine singular noun.

In the past tense, Russian verbs agree in gender and number with the subject:

• masculine: оказался
• feminine: оказалась
• neuter: оказалось
• plural: оказались

Since the subject is подъём, you get оказался.

Yes. Подъём is a noun meaning rise, lifting, ascent, or increase, depending on context. It is related to verbs such as:

• поднимать / поднять = to lift, raise
• подниматься / подняться = to rise, go up

In this sentence, подъём refers to the process or result of rising.

Yes. Out of context, теста can be the genitive singular of:

• тесто = dough
• тест = test

So подъём теста could look ambiguous by itself. In real life, the surrounding context tells you which meaning is intended.

Grammatically, though, the form works the same way in either case: it is still genitive singular after подъём.

The stress is:

• подъём — stress on ём
• тéста — stress on the first syllable
• оказа́лся — stress on а
• ме́дленным — stress on the first syllable

A useful point: ё is always stressed, even when writers sometimes replace it with е in casual writing.

So подъём is definitely stressed on the last syllable.

Russian has no articles. Words like a, an, and the are not used.

So Подъём теста оказался медленным can correspond to English versions such as:

• The rise of the dough turned out to be slow
• A rise in the test turned out to be slow

The exact choice in English depends on context, not on a separate article in Russian.

Yes. Russian word order is more flexible than English because the cases show the grammatical relationships.

The version here:

• Подъём теста оказался медленным.

is neutral and natural.

But other orders are possible for emphasis, for example:

• Медленным оказался подъём теста.
  This emphasizes slow.

Even though the word order can change, the cases still show that:

• подъём is the subject
• теста depends on подъём
• медленным is the complement after оказался

Медленный is the dictionary form: masculine singular nominative.

To make the instrumental singular masculine/neuter form, the ending changes:

• медленный → медленным

This is a regular adjective change.

So here:

• медленным agrees with подъём in gender and number
• but its case is instrumental because of оказался

That is why it is not медленного or медленный, but медленным.