Я не обиделся на друга, потому что это была только шутка.

Because обиделся is the perfective past form of обидеться, and it focuses on a single finished reaction: got offended / took offense.

In this sentence, Я не обиделся means I did not get offended in that situation.

If you said Я не обижался, that would sound more like:

• I wasn’t getting offended
• I didn’t used to take offense
• I wasn’t offended over some stretch of time

So не обиделся is the natural choice for one specific event.

The -ся shows that this is the verb обидеться, not обидеть.

Compare:

• обидеть кого-то = to offend someone
• обидеться на кого-то = to get offended at someone / to take offense at someone

So:

• Я обидел друга = I offended my friend
• Я обиделся на друга = I got offended at my friend

This is a very common verb pair in Russian: a verb without -ся affects someone else, while the -ся version describes what happens to the subject.

In the past tense, Russian verbs agree with the gender of the subject.

So:

• обиделся = masculine speaker
• обиделась = feminine speaker
• обиделось = neuter
• обиделись = plural

That means:

• Я не обиделся = said by a male speaker
• Я не обиделась = said by a female speaker

Russian past tense works this way very regularly.

After обидеться, Russian normally uses на + accusative:

• обидеться на кого? на что?

So друг changes to друга because it is in the accusative singular.

Examples:

• обидеться на брата
• обидеться на сестру
• обидеться на шутку

So in your sentence:

• на друга = at the friend / at a friend / at my friend, depending on context

This is just the standard pattern for this verb.

Russian has no articles, so nouns do not directly show a, the, or anything like that.

So на друга literally just means at friend in the accusative, and English has to choose:

• at a friend
• at the friend
• at my friend

The context tells you which one is most natural.

Also, Russian often leaves out possessives when they are obvious from context. So English may translate something more specifically than Russian states outright.

Because потому что introduces a subordinate clause meaning because.

Russian normally separates that clause with a comma:

• Я не обиделся на друга, потому что это была только шутка.

This is standard punctuation. The second part explains the reason for the first part.

This is a very common learner question.

Even though это often looks like something that should go with neuter было, in sentences like this the past-tense form often agrees with the noun that follows, especially when identifying what something was.

Here the noun is:

• шутка = feminine

So Russian says:

• это была шутка

not normally:

• это было шутка

Because шутка is feminine, the verb is была.

Other examples:

• Это был хороший день.
• Это была ошибка.
• Это было письмо.

Because in this kind of identifying sentence, the predicate noun is very naturally in the nominative:

• Это была шутка.

That is the normal, most straightforward way to say It was a joke.

You may also meet the instrumental in other contexts, for example:

• Это было шуткой

That can be possible in Russian, but it sounds different in nuance and is not the most natural basic version here. With это + was + noun, the nominative is very common and expected.

Because только usually goes right before the word or phrase it limits.

Here it limits шутка:

• это была только шутка = it was only a joke / just a joke

That placement makes it clear that the speaker is minimizing the situation: it was not something serious.

If you move только, the emphasis can change, and some positions sound much less natural.

Yes. Просто шутка is very common and often sounds very natural.

Compare:

• это была только шутка = it was only a joke
• это была просто шутка = it was just a joke / simply a joke

The difference is small here:

• только emphasizes only / merely
• просто emphasizes just / simply

In everyday speech, both are possible, though просто шутка may sound a bit more conversational in many contexts.

No, but it is the most direct and common choice.

Other possibilities include:

• так как
• поскольку

For example:

• Я не обиделся на друга, так как это была только шутка.

But these can sound a bit more formal or written, depending on context. For ordinary spoken Russian, потому что is the most natural default choice here.