cũng

Here are the essential rules for using cũng:

• Also / Too: The most fundamental use of cũng is to mean 'also' or 'too' in affirmative sentences, showing similarity between two different subjects or actions (e.g., Tôi cũng đi = I am also going).
• Placement: The fundamental structure is [Topic/Subject] + cũng + [Predicate]. While it often goes immediately after the subject and before the verb or adjective, it can also follow a time phrase, location phrase, or fronted topic. It must be placed before tense markers like đã, đang, sẽ and negation words like không or chưa. (e.g., Hôm nay tôi cũng bận = Today I am also busy).
• Me too: Because cũng cannot end a sentence, you must use dummy words like vậy or thế to agree with a previous statement without repeating the whole verb phrase (e.g., Tôi cũng vậy or Tôi cũng thế = Me too / Likewise).
• Negative sentences: When paired with không, it translates to 'either' or 'neither' (e.g., Tôi cũng không biết = I don't know either).
• Degree modifier: Native speakers often use it to mean 'quite' or 'fairly' (e.g., Phim này cũng hay = This movie is quite good).
• Softening statements: It is heavily used before verbs to soften a statement, show hesitation, or express a mild concession (e.g., Tôi cũng muốn đi, nhưng bận quá = I kind of want to go, but I'm too busy).
• Dilemmas: Using contrasting verbs in a '[Verb 1] cũng [Adjective], [Verb 2] cũng [Adjective]' structure expresses 'either way' or a dilemma (e.g., Tiến cũng dở, lùi cũng dở = Advancing is bad, retreating is bad too).
• Inclusive pronouns: It pairs with question words to mean 'every' or 'any' (e.g., ai cũng = everyone, gì cũng = everything, đâu cũng = everywhere, lúc nào cũng / khi nào cũng = always). You can also use the structure [Noun] + nào cũng to mean 'every' or 'any' of that noun (e.g., Người nào cũng thích = Everyone likes it, Món nào cũng ngon = Every dish is delicious).
• Acceptability: The common phrase cũng được means 'okay' or 'fine', and is frequently used as a standalone response meaning 'That works,' 'Fine by me,' or 'Okay' when accepting a suggestion or compromising. Combined with inclusive pronouns, it means 'anything is fine' (e.g., Ăn gì cũng được = Eating whatever is fine).
• Even if / Still: It is used in correlative structures like cho dù ... cũng ... to mean 'even if ... still ...' (e.g., Dù mưa tôi cũng đi = Even if it rains, I will still go).
• Starting sentences: You cannot use cũng as a standalone sentence connector like the English 'Also, ...'. To add a new point, use a phrase like Ngoài ra (Besides / In addition). However, cũng can start clauses in specific fixed expressions like Cũng may (là)... (Fortunately...) or Cũng vì / Cũng tại... (Also because...). Additionally, because spoken Vietnamese frequently drops pronouns (see Dropping The Subject), a sentence can technically start with cũng in casual replies where the subject is understood (e.g., Cũng hay = [It is] quite good, Cũng được = [That is] fine).
• Both / Neither: You can repeat cũng (cũng... cũng...) to list multiple inclusive traits (e.g., Tôi cũng thích trà, cũng thích cà phê = I like both tea and coffee). To say 'Neither... nor...', repeat cũng with không using the structure không... cũng không... (e.g., Tôi không ăn, cũng không uống = I neither eat nor drink).
• Possibility / Guesses: Using cũng nên at the end of a sentence expresses a guess, hesitation, or possibility (e.g., Biết đâu anh ấy sẽ đến cũng nên = Who knows, he might come).
• Comparisons of equality: Use cũng ... như ... to mean 'as ... as ...' (e.g., Tiếng Việt cũng khó như tiếng Trung = Vietnamese is as difficult as Chinese).
• cũng vs. đều: Use cũng to say 'also' when comparing to a previously mentioned subject. Use đều (both/all) to group a plural subject together (e.g., Họ đều đi = They all go). When 'also' applies to a plural group that all share a trait, cũng is frequently paired with the plural marker đều (e.g., Họ cũng đều đi = They also all go).
• Conceding a point: The structure [Adjective] thì cũng + [Adjective] is used to concede a point before introducing a contrast (e.g., Đẹp thì cũng đẹp, nhưng đắt quá = It is beautiful, but too expensive).
• Expressing 'Even': Cũng is the required secondary particle when expressing 'even [X]' using phrases like ngay cả or đến (e.g., Ngay cả tôi cũng không biết = Even I don't know; Đến trẻ con cũng hiểu = Even a child understands).

Important: Because it is an adverb, you cannot use it directly before a noun (e.g., Tôi mua táo và cũng cam ❌). While you can repeat the verb (Tôi mua táo và cũng mua cam), native speakers usually prefer using cả ... nữa to connect nouns when saying 'also' or 'too'. However, you can use the compound conjunction cũng như (as well as) to connect nouns (e.g., Tôi thích táo cũng như cam = I like apples as well as oranges).