Adding -ing to a verb can lead to a few grammatical possibilities:
- One possibility is the creation of a present participle which, in combination with the auxiliary verb "be", forms one of the continuous tenses. (For example, I am running, he was waiting)
- Another possibility is the creation of an adjective. (For example, a boring day, an exciting phone call)
- A third possibility is the creation of a gerund, which is a noun form that usually refers to an activity. (For example, Sleeping is good, I enjoy swimming)
Nouns like decision and suggestion are more abstract and do not refer to an activity. Both these words are classified as nouns due to their noun endings (-ion). In addition, both can be used together with a definite or indefinite article (a / the) while deciding and suggesting cannot. (Just by the way, there are a number of endings that signify that a particular word is a noun.)
The examples given in the question were interesting:
read = verb reading = gerund (e.g. I like reading)/ present participle (e.g. I am reading)/ adjective (e.g. reading room)
interest = verb (to interest sb/sth) OR noun (an interest in sb/sth) interesting = adjective
talk = verb OR noun (a talk on sth) talking = gerund (e.g. Talking is not always useful) / present participle (e.g. We have been talking to each other a lot) / adjective (e.g. a talking doll)
My final thought is that it is often quite difficult to generalise a rule in English. A lot depends on the context in which the part of speech occurs
