عندي ألم في هذا السن.

Literally, عندي means at me or with me. In Arabic, that is a very common way to express I have in the present tense.

So:

• عندي ألم = I have pain
• more literally = There is pain with me

Arabic often uses this kind of structure instead of a separate verb meaning to have.

In standard Arabic, there is no everyday present-tense verb used exactly like English have in sentences like this. Instead, possession is often expressed with words like:

• عند = at / with
• لدى = with / in the possession of

So عندي ألم is the normal Arabic way to say I have pain.

It is built into عندي.

The -ي at the end means me / my. So عندي already tells you that the sentence is about me.

That is why Arabic does not need أنا here. You can say أنا عندي ألم for emphasis, but عندي ألم is already complete and natural.

Yes. لديّ ألم في هذا السن is also correct.

The difference is mostly one of style:

• عندي is very common and natural
• لديّ often sounds a bit more formal or written

Both can mean I have pain in this tooth.

Because it is indefinite here.

ألم means pain rather than the pain. Arabic often uses an indefinite noun after expressions like عندي when introducing something a person has.

In fully vowelled formal Arabic, it would be ألمٌ with tanwīn, but short vowels are usually not written in normal text.

في usually means in. Arabic commonly uses it to show where pain is located in the body.

So:

• في هذا السن = in this tooth

That is a normal Arabic way to express the location of pain.

After demonstratives like هذا and هذه, standard Arabic usually uses a definite noun with ال.

So:

• هذا السن = this tooth
• هذا الكتاب = this book
• هذه السيارة = this car

This is a basic pattern in Arabic. So هذا سن would not be the normal standard form here.

Because س is a sun letter, the ل in ال is not pronounced.

So السن is pronounced:

• as-sinn

not:

• al-sinn

In the full phrase, you would say something like fī hādhā as-sinn.

The first sound is ع (ʿayn), a consonant that does not exist in English. It is pronounced deep in the throat.

A rough transliteration is:

• ʿindī

English speakers often find this sound difficult at first. That is normal. The important thing is to notice that it is a real consonant and not just a vowel.

In careful formal pronunciation, it would be approximately:

ʿindī ʾalamun fī hādhā as-sinni

A few notes:

• ألمٌ is nominative
• السنِّ is genitive after في
• in normal writing, these final short vowels are usually omitted

So learners usually see:

عندي ألم في هذا السن

Yes. سِنّ can also mean age, so context is important.

In this sentence, the context is pain, so the meaning is clearly tooth, not age.