差不多 / 几乎
Similarities
Both 差不多 and 几乎 work as adverbs meaning “almost”, “nearly”, “very close to”.
他汉语说得很好，差不多跟中国人一样
他汉语说得很好，几乎跟中国人一样
Either word works in this sentence.
1. With adjectives
差不多 can go directly before a monosyllabic adjective; 几乎 usually needs words such as 一样, 这么, 那么 or 了.
他们俩差不多高
他们俩几乎一样高
他比我几乎高了一头
2. With numbers
李老师差不多六十岁了
几乎六十岁 is not used — 几乎 does not go before a number.
3. As an adjective
姐妹俩性格差不多
几乎 cannot be used this way — it is never an adjective.
4. “Almost happened”
几乎 can express that something almost happened but in the end did not.
这件事我几乎忘了告诉你了
差不多 cannot be used with this meaning.
Quick comparison
| 差不多 | 几乎 |
|---|---|
| approximately, almost the same | almost; almost happened |
| can go before a number | cannot go before a number |
| can be an adjective | cannot be an adjective |
| cannot mean “almost happened” | can mean “almost happened” |
差不多 → almost, approximately, similar
几乎 → almost; almost happened, but didn't