What is the function of 先 in 我諗聽日先買飛嘅話一定買唔到 (cantonese)
先 here is short for 先至

先(first)

至(then)


先至 (first then)= till... then

我諗等到聽日(先)買飛嘅話一定買唔到
I think if we wait (till) tomorrow (then) to buy ticket we will certainly fail to buy any
The word &quot;等到&quot; (wait) is implied

Example of difference between &quot;先至/ 先&quot; (till...then) and &quot;先&quot; (first/ for now)
做好功課(先至/先)准睇電視 - (Till) finish all the homework (then) allowed to watch TV.
你地坐吓等陣(先) - you guys sit for a while and wait (first/ for now)

When &quot;...先&quot; is followed by a verb phrase, it is a short form of 先至, meaning &quot;till ... then&quot;

When 先 is at the end of a words clause, it means &quot;first&quot; or &quot;for now&quot;