Он не любит клясться.

Любить is the dictionary form, meaning the infinitive to like / to love.
In this sentence, the verb has to agree with он, so it becomes любит: he likes / he loves.

So the pattern is:

он любит + infinitive

Here, the infinitive is клясться.

After любить, Russian often uses an infinitive to say that someone likes or does not like doing something.

So:

• Он любит читать = he likes reading / he likes to read
• Он не любит клясться = he does not like swearing / he does not like to swear

This is a very common structure in Russian.

The ending -ся is the reflexive marker, but it does not always mean a literal self in English.

In клясться, you should learn the whole verb as one unit: клясться.
It is simply the normal verb form for this meaning.

So although -ся is historically reflexive, here you should not try to translate it word-for-word as swear himself. It is just part of the verb.

Usually no. This is an important nuance.

Клясться normally means to swear an oath, to vow, or to swear that something is true.

It is not the usual verb for to curse / use profanity. For that, Russian more often uses verbs like:

• ругаться
• материться
• сквернословить

So a learner should be careful not to assume that клясться automatically means profanity.

Because the negation applies to любит: he does not like doing this.

So the idea is:

He doesn’t like [to do X]

That is why Russian says:

Он не любит клясться

If you moved the negation to the infinitive, the meaning would change.
For example, Он любит не клясться would mean something more like he likes not swearing / he likes to avoid swearing, which is a different and less neutral idea.

Because клясться can be used on its own.

You can swear that something is true, or swear by something, but you do not have to include that extra information every time.

So клясться by itself is perfectly normal if the sentence only wants to mention the action in general.

Yes, often it can.

Russian frequently drops subject pronouns when the subject is clear from context. So in conversation, you might hear:

Не любит клясться.

That can still mean He doesn’t like to swear, if the context already makes clear who is being talked about.

Including он makes the subject explicit and can sound more neutral, clearer, or slightly contrastive depending on context.

The given order is the most neutral one:

Он не любит клясться.

But Russian word order is fairly flexible, and changing it can shift emphasis.

For example:

Клясться он не любит.

This puts more focus on клясться. It is still understandable, but it is less neutral and more expressive.

So the original sentence is the best basic pattern for learners.

This is about aspect.

• клясться = imperfective
• поклясться = perfective

After любить / не любить, Russian usually prefers the imperfective when talking about a general habit, preference, or repeated type of action.

So:

Он не любит клясться
means he does not like doing that in general.

If you used поклясться, it would sound more like a single completed act: to swear once / to make an oath. That is usually not what this structure is expressing.

The stress is кля́сться.

A useful learner note: the written ending -ться in infinitives is often pronounced roughly like -ца in normal speech.

So кля́сться is pronounced approximately like:

KLYAS-tsa

Also, the stress in любит is лю́бит.