If your book doesn't show you elementary stuff like time, i would suggest getting a second book or replacing that one...
anyway,
how do you know when it i -na
you KNOW in a lot of cases by the ending letter. for example, the majority of "i" adjectives actually end in ...... wait for it ....... surprisingly .... "い". by this i mean い and not the other kana like き し ち に etc. etc.
there are obviously a few exceptions to the rule, but i have learned to treat these exceptions as they are, when i meet them. for example:
きらい (な) (dislike // hate)
this adjectives ends with い but is actually a "な" adjective. don't get too hung up on why it is a "な" adjective, just learn that it is and move on is my suggestion. (right or wrong, that's what i do)
"い" adjective examples are:
あかい -- red
すごい -- great
こわい -- scary
むずかしい -- difficult
あたらしい -- new
ふるい -- old
かわいい -- cute
etc...
"な" examples:
げんき(な) -- healthy // fine
ばか(な) -- stupid
べんり(な) -- convenient
as you can see for these few examples, if you right them in romaji, they actually end in an "i" but not if you right them in kana.
ill right about time if no-one else does, i am at work now, so ill check later
y