I'd like some advice on Japanese grammar...

[starseeker]

Hey, I got a few more quick questions.

#1:
How do you spell dipthonghs?
I can't really find any site that explains it.

Japanese doesn't really have dipthongs, but they're spelled by putting two vowel kana together.

ai: a + i = あい

#2:
Is it true that most of the Japanese letters are signs for objects?
Are these pretty much the only signs you need to learn?

http://www.csse.monash.edu.au/~jwb/hiragana_b.jpg

Oh, and I'm aware that '' = voiced versions of other letters.

Kanji are the signs used for objects, nouns, adjectives etc. But you can't read Japanese using only kanji. Some words don't have a kanji and for verbs, kanji are only used for the stem. The tense is marked using hiragana, called okurigana.
Example:

食べます　＝　食べ　＋　ます
I eat = the kanji for 'to eat' + the okurigana 'masu' which makes it the present (non-past)