Yes you can say that for any exam, it doesn't have to be a written one. However your sentence should be "I have to take a dance exam tomorrow" or "I have a dance exam tomorrow". Another way to say this phrase would be "I have an exam". For example: "I have a dance exam tomorrow" "I have an English exam next week"

To 'let yourself in' about something means to put yourself in a difficult situation. I'm assuming this is an extract from a novel? It doesn't seem like a common term used these days, as I have never heard someone say this in person. I guess in this context, the author meant this character and their friends have gotten themselves in a difficult situation since coming from New York.

I have never heard of a gerund in English but I have looked it up and seems that just is a verb functioning as a noun so you add ing on the end. As for your sentences, yes 'wasting of time' sounds unnatural, instead try using 'wasting time' however, you used it correctly in sentence number 5. To answer your question, yes you can say a 'waste of time', instead of 'wasting time' both are correct. The reason you don't say 'a wasting' is because you don't put 'a' before a verb, it is grammatically incorrect. Words in English can also be verbs, nouns, adjectives, etc. depending on how they are used. For example 'waste' can be a noun, verb, adjective etc. 'Rubbish is another way of saying waste' (This uses waste as a noun) 'We need to take care of the environment and not waste water' (This uses waste as a verb) 'A wasting disease' (This uses waste as an adjective) I have included corrections to your sentences below: 1) Wasting time is a great problem of modern times - we use wasting here because it is present tense, while 'a waste of time' would be past tense. 2) This step prevents wasting time 3) Just wasting time and money - this sentence is correct but seems incomplete, 'you are just wasting time and money' sounds better to me. 4) GIS and MCDA were developed to minimize the wasting of space for the siting of hazardous waste landfills 5) You have to pay for it and you have to pay for it with your life, spending a lot of time, you pay for it with time, not the wasting of time but the spending of time - this sentence is correct. 6) She liked him calling her every evening.

All are correct but number 2 sounds the most natural for everyday language. 1 and 3 sound quite formal, you could use those in an academic essay but they do sound a bit unnatural in a normal discussion.

Depending on context you can say this in different ways. You don't need to include the "for three times", as this is not natural. For example: "I've read these books three times" "I've read those books three times" As above person mentioned you need a possessive noun before books (these, those, etc).

Yes this sentence is 100% correct. As the previous person mentioned, 'there' in this context does not mean the same thing as 'here'. 'There' is an adverb used (usually used at the beginning of a sentence) to introduce the subject, and in this case describe the location. For example 'There are many cats here', 'There is a nice smell coming from here', etc.