I really wonder what's the right way to make a verb to adjective? "inviteable" or "invitable"? "closeable" or "closable"?
Are there rules about these or it's just my decision which one to use?
Well, I've never seen "invitable" spelled "inviteable"; in fact, "invitable" is a very uncommon word, but it exists. I have seen "closable" spelled "closeable", so that's fine. I think there are just alternate spellings of words as you've seen above. In some instances, however, there are no alternate spellings. "Interchangeable" must always have the "e" before the morpheme -able; it cannot be spelled "interchangable". It would be wrong if it were spelled that way. Here are some alternate spellings of words:
aether = ether
aeon = eon
apnoea = apnea
colour = color
practise (verb) = practice (verb)
encyclopaedia = encyclopedia
apophthegm = apothegm
realise (verb) = realize (verb)
Some words have different spellings, but the pronunciations change when the spellings change:
spelt (verb) = spelled
leapt (verb) = leaped
dreamt (verb) = dreamed
leant (verb) = leaned
crew (verb) = crowed
shorn (verb) = sheared
This usually happens with verbs and not so much with other forms such as nouns and adjectives. The reason for this is that certain verbs have variant past-tense and past-participle paradigms.
As for a general rule, I can think of none off the top of my head except that, often, but not always, a soft g sound allows the e to remain:
swinge (verb) = swingeing
singe (verb) = singeing
But this is not always the case:
change = changing
cringe = cringing
The spelling rules of English are all over the place. You'll just have to learn how it's spelled and move on. There are many words with variant spellings, so that can both aid you by giving you more choices and leeway, but it can also hinder you because it can be confusing as well.
I hope that might have helped you out. Take care and good luck!