是因为...
Reach for this when you want to give the reason for something — 'It's because...' The previous sentence (or your conversation partner's question) sets up the situation; 是因为 then drops the cause cleanly at the start of the explanation. It's the Chinese version of starting a reply with 'Because' or 'It's because' in English: short, direct, and natural in speech. Most often used as a standalone reply or as the second half of a two-part statement.
Structure
[CONTEXT]. 是因为 [REASON]。
shì yīnwèi...
How to Think About It
是 here means 'it is' — it points back to whatever the conversation is about and labels the upcoming clause as the reason. So 是因为AI literally is 'It is — because of AI.' Without 是, just 因为 alone at the start of a reply feels like the front half of a 因为...所以... and your listener will be waiting for the 所以.
Examples
—你为什么不去？—是因为我太累了。
— Nǐ wèishénme bù qù? — Shì yīnwèi wǒ tài lèi le.
"Why aren't you going?" "It's because I'm too tired."
今年公司好，是因为我们有了新产品。
Jīnnián gōngsī hǎo, shì yīnwèi wǒmen yǒu le xīn chǎnpǐn.
The company is doing well this year — it's because we have a new product.
他没来上班，是因为他生病了。
Tā méi lái shàngbān, shì yīnwèi tā shēngbìng le.
He didn't come to work — it's because he got sick.
Common Mistake
Learners drop the 是 ('因为AI') because they remember 因为 as 'because.' In a standalone reply, 因为 alone is the front half of a longer structure and sounds incomplete; the 是 anchors it as a full statement on its own.
—公司为什么这么好？—因为AI。
—公司为什么这么好？—是因为AI。
Don't Confuse With
因为...所以...
Full cause-and-result structure in one sentence. Use 因为...所以... when you're stating both halves yourself; use 是因为 when the situation is already established and you're just providing the cause.
...的原因是...
Topic-first version — 'the reason for X is Y.' More formal and explicit. Use when you want to name the thing being explained; use 是因为 for casual conversational replies.
...，所以...
Result-half only — 'so X.' Use 所以 when the cause has just been stated and you're delivering the consequence; use 是因为 for the opposite direction (consequence first, then cause).
Practice
Fill in: —你为什么不去？—___因为我累了。
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是
Fill in: 今年公司好，是___我们有了新产品。
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因为
Arrange: 是 / 我 / 太累了 / 因为
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是因为我太累了。
Translate: He didn't come — it's because of the rain.
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他没来，是因为下雨。
Write a two-line reply using 是因为... explaining why something happened.
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—你为什么学中文？—是因为我喜欢中国文化。
Hear It in Real Episodes
This pattern appears in 1 Fluentide episode: