Он решил поддать пару.

Because it is formed from the prefix под- plus the verb дать.

• под-
  • дать → поддать
• One д belongs to the prefix, and one д belongs to the root.

This is normal in Russian spelling when a prefix ends with the same consonant that the root begins with.

No. Here поддать is not just дать with a simple literal meaning.

In this expression, поддать means something like:

• to add a bit more
• to intensify
• specifically in a bathhouse context, to throw water onto hot stones so that more steam rises

So поддать here is part of the idiomatic expression поддать пару.

The prefix под- often adds a sense like a bit more, upward, or to increase something slightly.

Here пару is a special form of пар meaning steam.

It is the so-called partitive form, sometimes called the second genitive. Russian sometimes uses -у / -ю with mass nouns when the meaning is some amount of something.

Compare:

• чаю = some tea
• сахару = some sugar
• пару = some steam

So поддать пару means literally something like to add some steam.

This is especially common in fixed expressions and colloquial usage.

No.

Even though пару can also be a form of пара meaning pair or a couple, that is not what is happening here.

In поддать пару, пару comes from пар meaning steam, not from пара.

The context and the fixed expression make that clear.

Решил is the past tense masculine singular form of решить.

It matches он:

• он решил
• она решила
• они решили

So the form tells you that the subject is:

• past tense
• singular
• masculine

This is a very common Russian pattern.

After verbs like:

• решить = to decide
• хотеть = to want
• планировать = to plan
• собраться = to intend/get ready

Russian normally uses an infinitive to say what someone decided, wanted, or planned to do.

So:

• Он решил поддать пару = He decided to do that action.

This works very much like English decided to + verb.

Because поддать is the perfective infinitive, and it fits the idea of one complete action.

In this sentence, the speaker is talking about:

• one decision: решил
• one bounded action: поддать

If you used поддавать, that would sound more like:

• doing it repeatedly
• doing it as a process
• doing it in a habitual way

So:

• решил поддать пару = he decided to do it once
• решил поддавать пару каждые пять минут = he decided to keep adding steam every five minutes

Because решить means to make a decision / come to a decision, which is a completed event.

So Он решил... means he reached the point of deciding.

If you said он решал..., that would mean something more like:

• he was trying to decide
• he was in the process of deciding

So the perfective решил is the natural choice if the decision was actually made.

Yes, very much so.

It is a familiar expression associated especially with a баня or sauna-like bathhouse setting. A learner should treat it as a set phrase rather than trying to interpret every part too literally.

You may also hear related forms in speech, especially colloquial ones.

So even though the grammar is explainable, the whole phrase is best learned as a chunk:

• поддать пару

It can be figurative too.

Literally, it refers to creating more steam in a bathhouse. Figuratively, it can suggest:

• increasing intensity
• livening things up
• pushing things harder
• turning up the heat, in a broad sense

But the literal bathhouse meaning is the core one, and that is usually the first meaning learners should remember.

Yes, if the subject is already clear from context.

Russian often leaves out subject pronouns when they are understood. So in the right context, you might hear simply:

• Решил поддать пару.

But in an isolated sentence, including он is completely natural and helps make the subject explicit.

The stress is:

• Он реши́л подда́ть па́ру.

A few pronunciation notes:

• реши́л has stress on the last syllable
• подда́ть has stress on -да́ть
• па́ру has stress on the first syllable

So the rhythm is:

• он реши́л подда́ть па́ру