What is the function of 同 in 我哋使唔使同佢準備個生日蛋糕？
Depend on the context, &quot;同&quot; as a preposition, can mean &quot;with&quot; or &quot;for&quot;


&quot;我哋使唔使[同]佢準備個生日蛋糕&quot; -- Do we need to prepare a birthday cake [for] him

&quot;我哋使唔使[同]佢(一齊)準備個生日蛋糕&quot; -- Do we need to prepare a birthday cake [with] him (together)



&quot;同&quot; strongly implies &quot;for&quot; in your example sentence. Birthday boy usually doesn't prepare the cake himself
More examples:
&quot;[同]我打電話叫警察&quot; = &quot;make a phone call to the police [for] me&quot;  (You don't need two people to make one phone call)
&quot;同我去釣魚&quot; is ambiguous, it can mean &quot;go fishing with me &quot; or &quot;go fishing for me&quot;. To eliminate the ambiguity, you can say &quot;同我一齊去釣魚 (go fishing with me together) or &quot;幫我去釣魚&quot; (go fishing for me).
Again with the context: If it is a friend saying to you: &quot;同我去釣魚好唔好?&quot;, it would clearly be an invitation to you to go fishing [with] him; If you are a slave and your master say to you:&quot;快啲同我去釣魚!&quot; , then it would clearly be an order to you to go fishing [for] him.
As you can see, when &quot;同&quot; means &quot;for&quot; you can replace it with &quot;幫&quot; (help/ for) e.g. &quot;[幫]佢準備個生日蛋糕&quot;, &quot;[幫]我打電話叫警察&quot;,