عندما كنت آكل، كان عندي ألم في هذا السن.

عندما means when in the sense of introducing a time clause: when I was eating.

In this sentence:

• عندما = when
• كنت آكل = I was eating

So عندما كنت آكل sets the time background for the main event.

A learner should know that عندما is very common in Modern Standard Arabic for this kind of sentence. You could also see other time words in Arabic, but عندما is a very standard choice here.

Because كنت آكل expresses an ongoing action in the past: I was eating.

Compare:

• كنت آكل = I was eating / I used to be eating
• أكلت = I ate

In this sentence, the speaker wants to show that the pain happened during the act of eating, not simply after a completed action. So the imperfect verb آكل with كنت gives that background, continuous sense.

This is very similar to the English difference between:

• When I was eating...
• When I ate...

They do two different jobs.

1. كنت آكل

   • كنت is كان conjugated for I
   • It helps form the past progressive meaning: I was eating

2. كان عندي ألم

   • كان here is he/it was
   • It means there was / I had

So although both come from كان, they are not repeating the same idea. One builds was eating, and the other introduces the state there was pain / I had pain.

Literally, it is something like:

• كان = there was / was
• عندي = at me / with me
• ألم = pain

So the literal structure is close to there was pain with me or there was pain at me.

But natural English is simply:

• I had pain
• or I felt pain

Arabic often uses possession-like structures with عند, especially in everyday-style expressions.

Because Arabic often expresses possession using a prepositional phrase rather than a verb like English have.

• عند = at / with
• ي = my / me

So:

• عندي = at me / with me = I have

Examples:

• عندي كتاب = I have a book
• كان عندي ألم = I had pain

This is very normal Arabic structure. English speakers often expect a direct equivalent of have, but Arabic commonly uses expressions like عندي or لدي instead.

Yes, كان لدي ألم is also correct.

The difference is mostly one of style:

• عندي is very common and natural
• لدي is often felt to be a bit more formal or written

So:

• كان عندي ألم = very natural
• كان لدي ألم = also correct, slightly more formal

In Modern Standard Arabic, both are acceptable.

Because the sentence means pain in a general sense, not the specific pain already known to both speaker and listener.

• ألم = pain / some pain
• الألم = the pain

With body symptoms, Arabic very often uses the indefinite noun:

• عندي صداع = I have a headache
• عندي ألم = I have pain

So كان عندي ألم sounds natural here.

Here في means in or in/at with reference to a body part.

• في هذا السن = in this tooth

Arabic often uses في to locate pain:

• ألم في ظهري = pain in my back
• ألم في بطني = pain in my stomach
• ألم في هذا السن = pain in this tooth

So even though English might sometimes prefer in or in this tooth, the Arabic structure is very straightforward.

Yes, سِنّ / السن can mean tooth, and it can also mean age. The context tells you which meaning is intended.

In this sentence:

• ألم في هذا السن clearly means pain in this tooth

because pain fits naturally with a tooth.

If someone said:

• في هذا السن يجب أن يدرس كثيرًا

  then السن would more likely mean age: At this age, he should study a lot.

So this is a good example of how context resolves ambiguity.

The word begins with س, and س is a sun letter. That means the ل of الـ is not pronounced separately.

So:

• السن is pronounced approximately as-sinn, not al-sinn

And in the full phrase:

• هذا السن ≈ hādhā as-sinn

This is an important pronunciation rule in Arabic spelling and reading.

Because سن is grammatically masculine in Arabic.

So:

• هذا السن = this tooth

If the noun were feminine, you would use هذه instead.

Even though many body-part words have to be memorized for gender, here the correct form is definitely هذا.

Yes. That would also be correct, but it means something slightly different in focus.

• كان عندي ألم في هذا السن = I had pain in this tooth
• شعرت بألم في هذا السن = I felt pain in this tooth

The first presents the pain more as a state or condition.
The second emphasizes the act of feeling it.

Both are natural, but كان عندي ألم is a very common way to describe having pain.

Because the whole sentence is talking about a past situation.

• عندي ألم = I have pain
• كان عندي ألم = I had pain

Since the first clause is already in the past (كنت آكل = I was eating), the second clause also needs to be in the past to match it.

So the sentence is fully anchored in the past:

• When I was eating, I had pain in this tooth.

Yes, it is grammatically correct and natural in MSA.

That said, depending on style, you might also encounter alternatives such as:

• عندما كنت آكل، شعرت بألم في هذا السن.
• عندما كنت آكل، أحسست بألم في هذا السن.

These alternatives focus more directly on feeling the pain. But the original sentence is perfectly good Arabic and very understandable.

In careful full vocalization, it would be:

عِنْدَمَا كُنْتُ آكُلُ، كَانَ عِنْدِي أَلَمٌ فِي هَذَا السِّنِّ.

A few notes:

• كنتُ ends in -tu because it is I was
• آكلُ has -u in normal nominative/indicative form
• ألمٌ is indefinite, so it takes tanwīn
• السِّنِّ has -i after في, because it is governed by a preposition

In ordinary Arabic writing, these short vowels are usually omitted, so learners often see:

عندما كنت آكل، كان عندي ألم في هذا السن.