Core difference
也 (yě) is an adverb that links the current sentence to a previous sentence or implied statement. It says "this subject is ALSO doing the thing we were just talking about". 都 (dōu) is an adverb that sums up across every member of the plural subject of its own sentence. It says "every single one of these does the action". 也 is comparative across clauses; 都 is distributive within a clause. That single framing explains 95 percent of usage differences.
Sentence structure
Both adverbs sit before the verb: Subject + 也/都 + Verb + Object. 也 typically has ONE subject: 我也喜欢 (I also like it). 都 typically has a PLURAL subject — multiple people or items listed before it: 他们都去 (they all go), 这些都是 (these are all). The position rule is firm — 也 and 都 never move after the verb, never before the subject.
我喜欢苹果。我也喜欢香蕉。／苹果、香蕉、橘子我都喜欢。
Wǒ xǐhuān píngguǒ. Wǒ yě xǐhuān xiāngjiāo. / Píngguǒ, xiāngjiāo, júzi wǒ dōu xǐhuān.
I like apples. I also like bananas. / Apples, bananas, oranges — I like them all.
Combining 也 and 都
The two can stack: 也都 means "also all (of them)". 他们也都来 = "they all come too" (on top of some previously mentioned group also coming). The order is fixed: 也 always precedes 都. Reversing it is ungrammatical. Use this combination sparingly — it is grammatically correct but can sound stilted in casual conversation.
学生都喜欢这个老师。老师们也都喜欢他。
Xuéshēng dōu xǐhuān zhège lǎoshī. Lǎoshīmen yě dōu xǐhuān tā.
The students all like this teacher. The other teachers also all like him.
Negation: 也不 vs 都不 vs 不都
Negation reveals a subtle but important split. 也不 = "also not" (I, too, do not do it). 都不 = "none of them do" (all of them do not). 不都 = "not all of them do" (some do, some do not). 我们都不去 means none of us are going; 我们不都去 means not all of us are going (some will, some will not). Learners often reach for 都不 when they mean 不都 — a common speaking error that changes the meaning entirely.
学生都不懂。／学生不都懂。
Xuéshēng dōu bù dǒng. / Xuéshēng bù dōu dǒng.
None of the students understand. / Not all of the students understand.
Quantifier triggers for 都
都 has a love-affair with 每 (every), 所有 (all), 大家 (everyone), 任何 (any) — these quantifiers almost always co-occur with 都 in the same sentence: 每个人都要来 (every person must come), 所有书都在这儿 (all the books are here). Missing the 都 after these quantifiers sounds incomplete to native speakers. 也 has no such dance partner; it stands on its own semantic link between clauses.
Native-speaker instinct
If the English sentence uses "also / too / as well", reach for 也. If it uses "all / both / every / everyone", reach for 都. If it uses both — "we all also like it" — stack them as 也都. The moment to pause and re-check is when the English sentence has "not all" or "none": those two map to 不都 vs 都不 in Chinese and flipping them changes the truth of your statement.