unless = if not

Unless is similar in meaning to if not and can be used instead of if not in certain types of conditional sentences. We normally use unless with present tenses when we are referring to the future:

You won't get in to see the show, if you don't have reserved seats. OR:
Unless you have reserved seats, you won't get in to see the show.

Let's play tennis on Saturday, if it's not raining. OR:
Let's play tennis on Saturday, unless it's raining.

I'll see you at the gym this evening, if you're not too tired. OR:
I'll see you at the gym this evening, unless you're too tired.

if not

However, we cannot use unless in questions:

• What will you do if you don't pass those exams?
  
• If I don't pass those exams, I won't be able to study in Australia
  
• I won't be able to study in Australia, unless I pass those exams.

And we cannot use unless with would to talk about unreal future situations:

• If he didn't take everything so seriously, he would be much easier to work with.
  
• If he weren't so bad-tempered, I would help him to get the work done

We cannot use unless with would have to talk about unreal situations in the past either:

• If you hadn't driven so recklessly, you wouldn't have had this accident.
  
• If you hadn't had that last glass of wine, this would never have happened.
  

unless

We have to use unless, and not if not, if we are introducing an idea as an afterthought:

• I shan't bother to go to the meeting at the school tonight - unless you want to go, of course.

Note that in written English, as regards punctuation, the afterthought is usually preceded by a dash.

unless + past participle

Unless can be used with a past participle in a reduced clause, Haja, when you choose to omit the subject words and the auxiliary verbs within the brackets in the examples below:

• Don't shut down these computers unless (you are) instructed to do so. Just log off.
  
• Unless (he is) given sufficient warning of the consequences, he will continue to misbehave.

However, this often makes the language produced sound rather formal and in spoken English we would normally retain subject words and auxiliary verbs.

otherwise = apart from this / if not

Otherwise is used as a linking adverb and has the meaning of apart from this or if we disregard this:

• The sea was very rough and we couldn't swim all week, but otherwise / apart from this we enjoyed ourselves.
• They all suffered from hypothermia. Otherwise, / Apart from that, they were OK.

It also has the meaning of if not, in the sense of if this does not happen, or if this were not the case, when it is used as a linking device:

• Remember to use sun cream with high protection when you go down to the beach. Otherwise, / If you don't, you'll get sunburnt within half an hour.
• Look, we really must hurry. Otherwise, / If we don't, we'll miss the train.
• He must be quite intelligent. Otherwise, he wouldn't have got into university. / If he wasn't, he wouldn't have got into university.