Он ждал её реакции.

Ждать is the dictionary form, meaning to wait or to expect.

In the sentence, you need a finite verb, so Russian uses the past-tense form ждал:

• ждать = to wait
• ждал = waited / was waiting

Because the subject is он (he), the past form is masculine singular:

• он ждал = he waited / he was waiting
• она ждала = she waited / was waiting
• они ждали = they waited / were waiting

So Он ждал её реакции literally has he waited for / expected her reaction.

Because Russian ждать usually takes a direct object, so you do not need a separate word like English for.

Compare:

• English: He waited for her reaction
• Russian: Он ждал её реакции

So Russian expresses the idea with the verb ждать + object, not wait + for + object.

Because after ждал, the noun is not in the dictionary form. It changes case.

The dictionary form is:

• реакция = reaction

In this sentence it becomes:

• реакции

Here that form is genitive singular.

So:

• реакция → nominative singular
• реакции → genitive singular

This is one of the trickier things about ждать: it often uses the genitive with what is being awaited, especially with abstract things like a reaction, answer, response, permission, etc.

This is a very common learner question, because English does not do anything similar.

With ждать, Russian can use either genitive or accusative, depending on usage and nuance. In many standard cases, especially with something abstract or anticipated, genitive is very natural.

So:

• ждать реакции = to wait for a reaction
• ждать ответа = to wait for an answer
• ждать помощи = to wait for help

In Он ждал её реакции, reaction is an abstract thing that has not happened yet, so the genitive sounds natural and standard.

A learner-friendly rule is:

• with ждать, be ready to see genitive, especially with abstract or expected things

You may sometimes encounter accusative with ждать, but in this sentence её реакции is the more standard and natural choice.

For a learner, the safest option here is:

• ждать + genitive for things like answer, reaction, help, news

So Он ждал её реакции is the form you should learn and trust.

Here её means her in the possessive sense:

• её реакция = her reaction

So in the sentence:

• Он ждал её реакции = He waited for her reaction

A useful thing to know is that её can also mean her as an object pronoun, but here the context shows it is possessive because it directly modifies the noun реакции.

Compare:

• Он видел её. = He saw her.
• Он ждал её реакции. = He waited for her reaction.

Same form, different function.

This is a subtle but important point.

The possessive pronoun её is indeclinable in modern standard Russian, which means it does not change form for case.

So even though реакции is in the genitive, её still stays её:

• её реакция
• её реакции
• её реакцию

The noun changes, but её does not.

It is genitive singular.

The noun is:

• реакция = reaction

Its singular cases include:

• nominative: реакция
• genitive: реакции
• accusative: реакцию

So in this sentence, реакции is genitive singular because of the verb ждать.

It can be either, depending on context.

Because ждать is imperfective, ждал often has an ongoing sense:

• he was waiting
• he waited
• he had been waiting

Russian past tense does not map one-to-one onto English past forms. The exact English translation depends on the wider context.

So Он ждал её реакции could mean:

• He was waiting for her reaction.
• He waited for her reaction.

If you want to understand the Russian, the key idea is that he was in the state of waiting/expecting.

Because ждать describes the process or state of waiting.

Russian aspect matters a lot:

• ждать = imperfective, focuses on the process/state
• perfective verbs would give a different meaning

For example, a perfective verb might suggest:

• he waited for a while and then stopped
• he managed to get the awaited result
• he waited until something happened

But ждал simply presents the waiting itself. That is why it fits naturally in a sentence like this.

Yes. Он ждал её реакции is a neutral, normal word order.

Structure:

• Он = subject
• ждал = verb
• её реакции = object

Russian word order is flexible, but this version is straightforward and unmarked.

You could change the order for emphasis, for example:

• Её реакции он ждал.

That would sound more emphatic, something like It was her reaction that he was waiting for.

But the original sentence is the basic neutral order.

You keep the object phrase the same and change the subject and verb form:

• Он ждал её реакции. = He was waiting for her reaction.
• Она ждала её реакции. = She was waiting for her reaction.
• Они ждали её реакции. = They were waiting for her reaction.

The past-tense ending changes to match the subject:

• ждал = masculine singular
• ждала = feminine singular
• ждали = plural

A rough pronunciation guide is:

• Он ждал её реакции
  on zhdal ye-YO ree-AK-tsi-i

A few points:

• ж sounds like the s in measure
• ё is always stressed: её
• реакции has four syllables: ре-ак-ци-и

If you want to sound more natural, try saying it smoothly as one phrase:

• Он ждал её реакции.