General synonyms:
* Speechless
* Mute
* Dumb (can be considered offensive, so use with caution)
* Uncommunicative
* Unspeakable
* Tongue-tied
* Wordless
* Silent
Synonyms emphasizing difficulty expressing oneself:
* Clumsy
* Awkward
* Fumbling
* Halting
* Rambling
* Confused
* Unclear
* Vague
Synonyms emphasizing a lack of verbal skill:
* Illiterate
* Ungrammatical
* Unpolished
* Crude
Synonyms emphasizing a lack of expression:
* Blank
* Empty
* Void
The best synonym for "inarticulate" will depend on the context and the specific meaning you want to convey.