Я клянусь говорить правду.

Клянусь is the 1st person singular form of клясться, which means to swear.

So:

• клясться = to swear
• я клянусь = I swear

This verb is a little irregular, so the form does not look exactly like the infinitive. A few present-tense forms are:

• я клянусь
• ты клянёшься
• он / она клянётся
• мы клянёмся

So in this sentence, клянусь is simply the verb form that matches я.

Because the dictionary form of the verb is клясться, a reflexive verb.

In Russian, reflexive verbs end in:

• -ся
• or -сь

After a vowel, Russian usually uses -сь, so:

• клянусь
• not клянусся

Here, the reflexive ending is just part of the verb. It does not mean the sentence is literally reflexive in the English sense, like I wash myself. Many Russian verbs are simply used in a reflexive form as part of their normal meaning.

So you should learn клясться as a whole verb meaning to swear.

Because the act of swearing happens right now.

In other words:

• клянусь = I swear now
• говорить правду = what I am swearing to do

This is similar to English:

• I swear to tell the truth

The verb swear is present in English too, even though the truth-telling will happen after the oath.

So the present tense here is completely normal. The sentence means that the speaker is making the oath now.

Because after клянусь, Russian commonly uses an infinitive to say what the person swears to do.

So the structure is:

• клянусь + infinitive

Here:

• клянусь говорить = I swear to speak / tell

This works much like English I swear to tell. Russian does not need a separate word for to before the infinitive.

So:

• Я клянусь говорить правду = literally I swear tell truth
• natural English: I swear to tell the truth

Because правду is the accusative singular form of правда.

• правда = truth
• правду = truth, as a direct object

In говорить правду, the noun is the thing being spoken/told, so Russian uses the accusative case.

This is a very common expression:

• говорить правду = to tell the truth

Compare:

• Это правда. = That is the truth.
  Here правда is nominative.
• Я говорю правду. = I am telling the truth.
  Here правду is accusative.

Both are possible in Russian, but they are not exactly the same.

• говорить правду uses the imperfective verb
• сказать правду uses the perfective verb

The difference is roughly:

• говорить правду = to speak/tell the truth in general, or throughout what you say
• сказать правду = to tell the truth once, to say the truth as a single completed act

In an oath, говорить правду is very natural because it suggests ongoing truthful speech, not just one single statement. It fits the idea of giving truthful testimony.

So:

• Я клянусь говорить правду = I swear to tell the truth
• Я клянусь сказать правду would sound more like swearing that you will reveal the truth in one specific statement

Yes. That is completely natural in Russian.

Russian often leaves out subject pronouns when the verb ending already makes the subject clear. Since клянусь already means I swear, я is not strictly necessary.

So both are possible:

• Я клянусь говорить правду
• Клянусь говорить правду

The version with я can sound a little more explicit or emphatic. Without я, the sentence is still clear and often sounds slightly more natural.

It sounds formal and solemn.

Клянусь is a strong word. It is used in situations involving:

• oaths
• promises made very seriously
• legal or ceremonial language
• dramatic or emotional speech

So Я клянусь говорить правду sounds like something you might hear in:

• a courtroom
• a formal oath
• a serious dramatic scene

In casual everyday speech, people usually choose less solemn verbs, depending on what they mean, such as обещаю for I promise.

The stress is:

• Я кляну́сь говори́ть пра́вду.

A rough pronunciation guide:

• Я = ya
• кляну́сь = klya-NOOS'
• говори́ть = gava-REET'
• пра́вду = PRAV-du

A couple of useful notes:

• The stress in кляну́сь is on the last syllable.
• The stress in говори́ть is also on the last syllable.
• The stress in пра́вду is on the first syllable.
• Unstressed о in Russian is often pronounced more like a, which is why говорить sounds closer to gavarit' than to govorit'.