At this site there's not only some explanation, but also a few exercises and answers to check if you really understood everything.

About de en het

Words like maar, eens and even

Constructing sentences

Nightmares of er

Verbs – general

Verbs – present

Verbs – past (regular)

Verbs – perfect (regular)

Irregular verbs

Verb couples

Pronunciation & spelling

On seperable and non-seperable verbs

On plural

Pronouns

Reflexive verbs

Comparative & superlative