Can you explain the grammatical structure of this sentence
Ik - I
ben - am
blij - glad
"Ik ben blij" is the main clause.
dat - that
ik - I
je - you
niet - not
vergeten ben - have forgotten
"Dat ik je niet vergeten ben" is the dependent clause.
A "dat" clause kicks the verb to the end of the clause; well, at least the verb has to come after pronouns and the negative particle "niet."
"Ben vergeten" is composed of the helping verb "ben" and the past participle "vergeten." It is also possible to conjugate "vergeten" with "hebben" (so you would say "ik heb vergeten"), but the two are not interchangeable. See
this page (in Dutch) for more details.
In a "dat" clause, "vergeten" comes before "ben."
If you have more questions, do let me know.
and how would you translate"I'm glad that I don't forget you." then?
Jippie already answered this question. It's the same thing, except that instead of "vergeten ben" you use the present tense ("vergeet").