"The book is big."
Translation:La libro estas granda.
Granda (big) is an adjective (a word that describes a noun). All adjectives in Esperanto end with an 'a'. Present tense verbs (words that represent a current action) end in 'as', thus estas (is), legas (read) and skribas (write). The base form (infinitives) of these are: esti, legi, skribi; meaning to be, to read, to write. Infinitives in Esperanto end in 'i'. As you can see in English it is easy to spot an infinitive by the word 'to'. "Grandas" cannot be a word, as you cannot use 'big' as a verb.
If there's different info somewhere, please link it for me. I checked as many online dictionaries as I could find and didn't find a "grandas" which with the -as ending would be a present tense verb. The infinitive would be "grandi"; I didn't find such a word anywhere either. There is according to http://en.lernu.net the word "grandigi" which means "to make big". I as far as I can tell all of this makes sense as "big" is an adjective not a verb, so "granda" is an adjective, easy to tell from the -a ending. Very much a beginner still, so I may be missing something, I'd like to know what though. :)
1) I'm not sure why you are comparing grandas to grandigi, as they have two different meanings. Grandigi means to make big [transitive]. Grandas means is big/great/large, the same as estas granda** [intransitive]. (Well, it depends on who you listen to—DuoLingo says in the Tips and Notes for Adjectives they mean the same, while Claude Prion says that "<adjective>-as" is more 'active' than "estas <adjective>".)
2) There's some information about verb-ing adjectives in the Wikipedia article on Esperanto grammer.
3) The Tekstaro de Esperanto is a text corpus of Esperanto gathered by the “Esperantic Studies Foundation”. If something is in there, I would say it's "correct usage" from a grammer perspective.
Unfortunately, this just shows that 'grandas, etc' has been used before, not if this is considered correct usage. :) According to the link, 'grandis/grandas/grandos' does seem uncommon though. The frequency of these words (including mal- and pli- forms of these words) is 4/19/0. By comparison, the frequency of 'grandigis/grandigas/grandigos' is 71/38/24. I would have thought, seeing as people seem to be inclined towards using 'grandis/grandas/grandos' that those who have much more knowledge and experience in Esperanto have discussed this usage. Sadly, my search skills in this have failed. Still, unless I find more definitive information on this, I do not intend to use these words/spellings or to encourage others to do so. It just seems too much like a snipe hunt.
The n goes at the end of a noun that has the action being done to it. I am not sure if it needs to be at the end of a sentience like in English. In English the subject (noun) does something (verb) to the object (noun) of the sentience. The order of the words dictate what the object is. In Esperanto, the n at the end of the word identifies the object. Example: The boy waxes the car. The car is the object. What if you switched the words boy and car? the car can't wax the boy. In Esperanto, the n helps identify that the car was the one getting waxed, not the boy. I am not sure how much word order is important in Esperanto. Sorry.