Теплоотдача стала слабой.

Because after стать in the meaning to become, Russian normally uses the instrumental case for the thing someone or something becomes.

So:

• слабая = nominative feminine singular
• слабой = instrumental feminine singular

That is why you get:

• Теплоотдача стала слабой. = Heat transfer became weak.

This is a very common pattern:

• Он стал врачом. = He became a doctor.
• Она стала известной. = She became famous.

So the key idea is: стать + instrumental.

Because the subject, теплоотдача, is a feminine singular noun.

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

• стал = masculine singular
• стала = feminine singular
• стало = neuter singular
• стали = plural

Since теплоотдача is feminine, the verb must be стала.

Here стала is the past tense of стать, and it means became.

So this sentence is about a change of state:

• Теплоотдача стала слабой. = Heat transfer became weak.

This is different from была:

• Теплоотдача была слабой. = The heat transfer was weak.
• Теплоотдача стала слабой. = The heat transfer became weak.

So стала emphasizes that it changed and ended up in that condition.

It is in the nominative case, because it is the subject of the sentence.

You can identify it by asking: What became weak?

Answer: теплоотдача

So:

• теплоотдача = subject, nominative
• слабой = predicate adjective in the instrumental after стать

It is a compound noun:

• тепло = heat
• отдача = giving off, output, release

So literally it is something like heat release, heat dissipation, or heat transfer, depending on context.

In technical or scientific contexts, теплоотдача often means:

• heat transfer
• heat dissipation
• heat emission

The best English translation depends on what kind of system is being discussed.

In this sentence, слабой is the normal and expected form because of the pattern стать + instrumental.

A short form like слаба exists, but it would not normally be used after стать in this way.

Compare:

• Теплоотдача слаба. = The heat transfer is weak.
  This is more formal/bookish and states a condition.
• Теплоотдача стала слабой. = The heat transfer became weak.
  This describes a change.

So for this sentence, слабой is the natural choice.

Russian has no articles.

So Russian does not have separate words for:

• a
• an
• the

Whether the meaning is definite or indefinite is understood from context.

So Теплоотдача стала слабой could mean:

• The heat transfer became weak
• Heat transfer became weak

The exact English wording depends on the situation.

Yes, Russian word order is fairly flexible, though not all orders sound equally neutral.

The most neutral order here is:

• Теплоотдача стала слабой.

You could also say:

• Слабой стала теплоотдача.

This puts more emphasis on слабой, something like It was weak that the heat transfer became or more naturally The heat transfer became weak with special focus on the result.

So the original order is the most straightforward and neutral.

The standard stress is:

• теплоотда́ча стала́ слабо́й

A rough pronunciation guide:

• teep-lah-ot-DA-cha sta-LA slab-OY

The important stressed syllables are:

• отда́ in теплоотда́ча
• ла́ in стала́
• бо́й in слабо́й

Yes, very much so.

The noun теплоотдача is common in technical, engineering, medical, and scientific contexts. So the sentence sounds like something you might read in:

• an engineering report
• a physics text
• a medical description
• a technical diagnosis of equipment

In everyday conversation, people are less likely to use теплоотдача unless they are talking about something specific like a radiator, a body’s heat loss, or a machine.

Yes. Depending on context, слабой could be translated as:

• weak
• poor
• low
• inefficient

For example, in a technical context, English often prefers:

• Heat transfer became poor.
• Heat dissipation became weak.
• The heat output became low.

So weak is the direct translation, but the best English version depends on the subject matter.