wǒ zhǐ shì zài kāiwánxiào.

A useful way to read it is:

• 我 = I
• 只是 = just / only / merely
• 在 = marks an ongoing action
• 开玩笑 = joke / kid

So the whole sentence means I’m just joking or I was only kidding, depending on context.

For learning purposes, it is best to treat 只是 here as one chunk meaning just / only / merely.

Even though it is historically made of 只 + 是, in this sentence you do not need to interpret 是 as the normal verb to be. If you learn 只是 as a set expression here, the sentence becomes much easier to understand.

Also note: 只是 can mean however / but in other sentences, but that is not the meaning here.

在 before a verb often marks an action as ongoing, similar to -ing in English.

So:

• 开玩笑 = to joke
• 在开玩笑 = to be joking

That is why 我只是在开玩笑 feels like I’m just joking rather than a more general statement like I joke.

Because this sentence is not using 是 as a normal linking verb like A is B.

The main action here is 开玩笑. The phrase 只是 works like an adverb meaning just / only. So it is better not to translate the sentence word-for-word as if 是 were a separate to be verb.

In other words, this is not the same kind of 是 as in:

• 我是老师 = I am a teacher

In Mandarin, adverbs usually come before the verb phrase they modify.

Here, 只是 modifies the whole action 在开玩笑, so it comes before it:

• 我只是在开玩笑

This is the normal word order for saying I’m only/just joking.

开玩笑 is a very common expression meaning to joke, to kid, or sometimes to mess around.

Literally, the parts are:

• 开 = open / start
• 玩笑 = joke

But you should learn 开玩笑 as a fixed expression, not as a literal phrase.

Examples:

• 别开玩笑。 = Don’t joke around.
• 你在开玩笑吗？ = Are you kidding?

Yes. That depends on context.

Chinese does not mark tense the same way English does. 在 shows an ongoing action, but it does not by itself force the sentence to be present tense in the same way English does.

So in conversation, 我只是在开玩笑 can be understood as:

• I’m just joking
• I was just kidding

If you want to make the past clearer, you can add a time word:

• 刚才我只是在开玩笑。 = I was just kidding just now.

Yes, absolutely.

• 我只是开玩笑 = natural
• 我只是在开玩笑 = also natural

The version with 在 makes the joking feel more immediate or in progress. The version without 在 is a little simpler and more general.

In many real situations, both are fine.

It is neutral and very common in everyday speech.

You can use it when:

• someone takes your words too seriously
• someone gets offended
• you want to soften what you just said

It is not especially formal, but it is perfectly standard Mandarin.

No. The sentence is already complete as it is.

• 了 is usually not needed here and may change the nuance.
• 呢 can be added in conversation to sound a little softer or more immediate:

我只是在开玩笑呢。

That sounds conversational, but the plain sentence 我只是在开玩笑。 is already correct and natural.