Hi, alc24! I'll try to help you with these sentences. Are these sentences that you are trying to write correctly for yourself or for an assignment?
1 When/If I don't have to wait, I take the subway otherwise I take a cab.
When is the better choice because you are talking about time, in reference to waiting. However, you also need to make this into two sentences or add a semicolon.
When I don't have to wait, I take the subway. Otherwise, I take a cab.
When I don't have to wait, I take the subway; otherwise, I take a cab.
2 I don't have to take a specific/particular subway as all three lines go through/past my stop. I can take whichever/whatever subway I want.
Specific refers to the line; particular refers to the individual subway; you can't choose the particular one because you don't know which one will show up, but you can choose a specific line. All lines go past your stop; if they went through it, they would knock everyone off the platform. Whichever refers to choosing from a few options, and whatever refers to choosing from many options.
3 When it suits/is convenient for you, he's your dog and when you don't feel like taking him out, he's my dog.
You could say either one; "When it suits you" implies when you feel like it whereas "when it's convenient for you" implies when it works easily into your schedule.
4 He speaks in abbreviations. CAN YOU SAY THAT? (ex : I'm in Aus at the mo, and won't be back to england for a lil.
Do you mean can you write it or say it? Certainly, abbreviations are common on social media sites such as Twitter, but you wouldn't talk to someone that way and you wouldn't write those abbreviations in a more formal letter or article.
5 This actor can do comedies just as well as he can do...
Hope these are helpful to you!
Lisa Scott