Cantonese 先 (sin1): first goes at the end
English puts first in front of the verb, and so does Mandarin. Cantonese hangs 先 sin1 on the end of the whole verb phrase, which is one of the clearest differences between the two spoken languages.
先 closes the phrase
Everything else in the sentence finishes first. The verb, the object, all of it lands, and then 先 goes on the end to say this one comes before whatever else is coming. It is the position that carries the meaning, not the word itself.
This is a real difference rather than a matter of style. Mandarin puts the word before the verb, 我先走. Cantonese puts it after the verb phrase, 我行先. Anyone arriving from Mandarin produces 我先行 for months before the habit shifts, and it is one of the fastest things to fix because the rule has no exceptions worth worrying about.
Not the 先 in 先至
先至 sin1 zi3 means only then. It sits in front of the verb in the second half of a sentence and waits on a condition: 食完飯先至去, not going until the meal is finished. That is about one thing depending on another, and it has its own page. The 先 here is about order alone: this one now, the rest after.
Where you will actually hear it
先 turns up constantly in ordinary politeness. Letting someone through a door is 你行先. Excusing yourself from a table is 我食飯先. Asking for a moment before you answer is 我諗吓先. It also softens an instruction, which is why it sits comfortably alongside the patterns on the requests page.
Common mistakes
Fronting it
我先食飯 is the Mandarin order. Spoken Cantonese closes the phrase instead: 我食飯先.
Confusing it with 先至
先 on the end orders two things. 先至 in front says the second one will not happen until the first does. Different jobs, different slots.
Adding a particle after it
先 is already closing the sentence. 我食飯先啦 is heard, but a bare 我食飯先 is the neutral form to learn first.
