費事

1. The Conjunction Function
The most common use of 費事 (fai3 si6) is to link an action with a negative outcome you want to prevent. It can be followed by either a full clause (Subject + Verb) or just a single verb/adjective. The structure is:
[Action] + 費事 + [Bad Outcome]
For example: 帶把遮啦，費事落雨呀 (daai3 baa2 ze1 laa1, fai3 si6 lok6 jyu5 aa3) — 'Take an umbrella, in case it rains.'
When the subject is the same as the main clause, it is often omitted, leaving just a verb or adjective: 著多件衫，費事凍親 (zoek3 do1 gin6 saam1, fai3 si6 dung3 can1) — 'Wear more clothes, lest [you] catch a cold.'

2. Sentence-Initial and Standalone Usage
You can start a sentence with 費事 to mean 'Lest...' or 'Just so you don't [complain/misunderstand]'. For example: 費事你話我唔幫你 (fai3 si6 nei5 waa6 ngo5 m4 bong1 nei5) — 'Lest you say I don't help you.' It can also be used alone as 費事啦 (fai3 si6 laa1) to mean 'I'd rather not' or 'It's too much hassle.'

3. The Adverb/Verb Function
It can also function as an adverb or verb meaning 'can't be bothered' or 'to spare the effort.' When used this way before another verb, it carries a blunt or dismissive tone, implying the speaker doesn't want to waste effort on something. It is highly frequent with verbs of communication or inquiry, such as 費事講 (fai3 si6 gong2 - 'can't be bothered to say') or 費事問 (fai3 si6 man6 - 'can't be bothered to ask'). (Note that the conjunction usage in Point 1 does not carry this rude or dismissive tone; it is simply a pragmatic way to say 'to save us the trouble of...').
Example: 我費事理佢 (ngo5 fai3 si6 lei5 keoi5) — 'I can't be bothered to deal with him.'

4. Register and Variants
費事 is strictly colloquial. In formal writing, you should use 以免 (ji5 min5) or 免得 (min5 dak1) for the conjunction usage ('so as to avoid'). For the 'can't be bothered' usage, the Standard Written Chinese equivalent is 懶得 (laan5 dak1). In digital communication, you may often see the informal variant character 廢事 (fai3 si6). Be careful not to confuse it with the actual word 費時 (fai3 si4), which means 'time-consuming.' However, because they sound so similar, native speakers frequently write 費時 instead of 費事 online (e.g., 費時理佢 fai3 si4 lei5 keoi5). While it often starts as a phonetic typo, many native speakers actually parse 費時 semantically as 'wasting time' (an eggcorn), rather than dismissing it as just a typo.

5. Mandarin False Friend Warning
For Mandarin speakers: In Mandarin, 費事 is primarily used as an adjective meaning 'troublesome' or 'inconvenient'. In Cantonese, 麻煩 (maa4 faan4) is overwhelmingly preferred instead. 費事 primarily functions in the conjunction/adverb roles described above (though you might occasionally hear it as an adjective, like 做呢樣嘢好費事 zou6 ni1 joeng6 je5 hou2 fai3 si6).

6. No Double Negatives
Because 費事 already implies 'avoiding' something, you do not need to add a negative word like 唔 (m4) after it to express a negative outcome. You only use a negative word if you specifically mean to avoid not doing something (e.g., 費事唔記得 fai3 si6 m4 gei3 dak1 'to avoid forgetting / in case I forget'). For example, you do not need to say 費事唔落雨 to mean 'in case it rains'.