Он пошёл на стажировку не потому, что его заставили, а потому, что хотел получить опыт.

Пошёл is the past tense of пойти, a perfective verb of motion. Here it means something like went / set off / went to do.

Why not шёл?

• шёл is from идти and usually describes the process: was walking / was going
• пошёл often focuses on the start or completion of the action as a whole

In this sentence, Он пошёл на стажировку means he went to do an internship / went off to an internship, not that he was physically in the middle of walking somewhere.

So the perfective verb fits well because the sentence is about the fact that he made that decision and did it.

Russian often uses на + accusative for going to events, activities, programs, or institutions in their functional role.

So:

• на работу = to work
• на учёбу = to study / to classes
• на стажировку = to an internship / for an internship

Even though English says on an internship or to do an internship, Russian idiomatically uses на стажировку.

This is just the normal collocation:

• пойти на стажировку
• устроиться на стажировку
• взять кого-то на стажировку

Because the preposition на can take different cases, and here it expresses direction toward something.

• на + accusative = motion toward / going onto / entering an activity
• на + prepositional = location at / on

Compare:

• Он пошёл на стажировку. = He went on an internship.
• Он был на стажировке. = He was on an internship.

So:

• на стажировку = destination or goal
• на стажировке = location/state

This is a very common Russian pattern meaning:

not because ..., but because ...

In your sentence:

• не потому, что его заставили = not because he was forced
• а потому, что хотел получить опыт = but because he wanted to gain experience

A useful thing to remember is that потому что by itself means because, but in this contrast pattern it gets split into two parts:

• не потому, что ...
• а потому, что ...

So the sentence is built as a contrast between a false reason and the real reason.

Because что introduces subordinate clauses.

The structure is:

• не потому, что его заставили
• а потому, что хотел получить опыт

In Russian, subordinate clauses introduced by что are normally separated by commas.

So the commas here are standard and required:

• before что after потому
• before а introducing the contrast
• again before the second что

This is normal punctuation, not optional style.

Because заставить takes the person being forced in the accusative case.

So:

• заставить кого? = to force whom?
• его заставили = they forced him / he was forced

Why not он?

• он is nominative, used for the subject
• here he is the object of заставили

Why not ему?

• ему is dative, but заставить does not normally use dative for the person forced

Examples:

• Меня заставили уйти. = They forced me to leave.
• Его заставили учиться. = They forced him to study.

This is a very common Russian pattern: 3rd person plural with no expressed subject.

It often means:

• they did X
• but without specifying who they are

In English, depending on context, we might translate it as:

• they forced him
• he was forced
• someone forced him

So его заставили literally looks like they forced him, but the point is not who did it. Russian often uses plural verbs this way when the agent is unknown, unimportant, or deliberately left vague.

Because Russian often omits a repeated subject when it is clear from context.

Full version:

• ...а потому, что он хотел получить опыт.

Natural shortened version:

• ...а потому, что хотел получить опыт.

Since the subject of both parts is clearly the same person, Russian does not need to repeat он.

English usually prefers repeating the subject:

• ...but because he wanted to gain experience.

Russian is more comfortable leaving it out when there is no ambiguity.

Because получить is perfective and here it means to obtain / gain experience as a result.

After хотеть, Russian can use either perfective or imperfective infinitives, but the choice changes the nuance:

• хотел получить опыт = wanted to gain/acquire experience
• хотел получать опыт = wanted to be gaining experience / wanted to gain experience repeatedly or as an ongoing process

In this sentence, the idea is the goal or result of doing the internship: to gain experience. That is why получить sounds natural.

Опыт means experience.

Here it refers to practical or professional experience, not an “experience” in the sense of a single event.

So:

• получить опыт = to gain experience
• получить опыт работы = to get work experience

Russian does not have articles, so опыт can mean:

• experience
• some experience
• practical experience

The exact English wording depends on context.

Yes. Russian word order is flexible, though not random.

The neutral version here is:

• Он пошёл на стажировку не потому, что его заставили, а потому, что хотел получить опыт.

But other versions are possible for emphasis, for example:

• Не потому, что его заставили, он пошёл на стажировку, а потому, что хотел получить опыт.
• На стажировку он пошёл не потому, что его заставили, а потому, что хотел получить опыт.

These alternatives shift emphasis:

• putting на стажировку first highlights the internship
• putting не потому, что... first highlights the contrast of reasons

For learners, the original order is the safest and most natural to use.

It can suggest either, depending on context.

Literally, it is went on/to an internship, but in real usage it often implies entering that stage or program, so in English it may sound natural as:

• He went on an internship
• He took an internship
• He started an internship

Russian often uses motion verbs in places where English might prefer a more abstract verb like start, take, or do.

So even though пошёл literally comes from go, the overall meaning can be broader than just physical movement.