因为...
Use 因为 by itself when you want to give a reason without the full 因为……所以…… frame. It's how you answer 为什么 questions in one short clause, or tack on an explanation after the result has already been stated. Functionally identical to a standalone English 'because' clause. At HSK 1, this lighter form shows up more in speech than the textbook pair, because real conversation rarely needs both halves.
Structure
因为 [REASON]
yīnwèi...
How to Think About It
Standalone 因为 is the reason clause cut loose from its 所以 partner. Chinese normally keeps both halves, so dropping 所以 only works in two situations: answering a 为什么 question (the result is already in the air) or attaching the reason after the main sentence as an afterthought. Lead a fresh sentence with just 因为 in writing and it sounds half-finished.
Examples
我没来，因为我生病了。
Wǒ méi lái, yīnwèi wǒ shēngbìng le.
I didn't come, because I was sick.
为什么不吃？因为不好吃。
Wèishénme bù chī? Yīnwèi bù hǎochī.
Why aren't you eating? Because it doesn't taste good.
我喜欢这个城市，因为人很好。
Wǒ xǐhuan zhège chéngshì, yīnwèi rén hěn hǎo.
I like this city, because the people are nice.
Common Mistake
Beginners drop 所以 in declarative sentences where they really need the full pair. Standalone 因为 only sounds natural as an answer or trailing afterthought, not as the opener of a written cause-and-effect sentence.
因为下雨，我没去。
因为下雨，所以我没去。
Don't Confuse With
因为...所以...
Use the full pair when you're writing or stating both cause and effect in one sentence. Standalone 因为 fits answers and tail-end explanations.
由于...
由于 is the formal, written version. Stick with 因为 in speech and casual writing.
...的原因是...
Use 的原因是 to put the topic first and then explain ('the reason is...'). 因为 leads with the cause directly.
Practice
Fill in the blank: 他没去上班，___他病了。
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因为
Fill in the blank: 为什么你这么开心？因为___考试考好了。
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我
Arrange: 我 / 因为 / 累 / 想 / 睡觉 / 了
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我想睡觉，因为我累了。
Translate to Chinese: I like her because she is kind.
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我喜欢她，因为她很好。
Use 因为 to answer the question: 你为什么学中文？
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因为我想去中国工作。