interdīco, interdīcere, interdīxī, interdīctus: forbid
Interdīco (forbid) gets a note of it’s own in A&G because it’s case constructions have varied over time.
- Earlier writers present interdīco + dative Person & ablative Thing Forbidden
- Later writers use interdīco + dative Person & accusative Thing Forbidden
Exempla
- They forbade him fire and water: aquā et īgnī eō interdīxērunt.*
- Shall we forbid the women from wearing purple: fēminīs purpurae ūsū interdīcēmus?
- He forbade the actors from appearing on the stage: histriōnibus scaenam accedere interdīxit.
*This was the standard formally for expressing ‘he is banished’
Also, I discovered during the construction of this post that ‘forbid’ is never the past tense of the English ‘forbid.’ It is usually ‘forbade’ and rarely ‘forbad.’ I hope I wasn’t the only person making this mistake… for 21 years…
The Essential AG: 365n1
Latin for Addicts 
Er, should that be interdictum (rather than interdixus)?
I’ve heard mixed arguments on the fourth principal part of Latin verbs: some insist it should be the neuter passive participle, others make it the masculine nominative. A&G (172) state that it should be the neuter. On the other side, it’s not at all difficult to find people using the masculine:
http://ancienthistory.about.com/od/verbs/f/021311-What-Are-The-Principal-Parts-Of-Latin-Verbs.htm
http://wiki.answers.com/Q/What_are_the_principal_parts_of_a_Latin_verb
Ultimately, it’s a debate about whether the fourth principal part should be the passive participle or the supine. The one trouble with using the neuter (technically the supine) is that where no supine exists, manuals like A&G (172) say we should use the future active participle instead. I’m lazy, so I use the masculine passive participle, because I know it will always exist.
Could you give an example of a verb that lacks a supine but does have a perfect passive participle?
I always figured if the 4th principal wasn’t listed any perfect system passive idea is impossible.
A&G mean it as ‘not existing in the standing body of Latin text’ —i.e. if it never appears in anything we have, they want to assume that may be because it was awkward as a supine, and so never used as a supine. Therefore, if verb x has no supine extant in all Latin literature, rather than assume it exists, we use the future active participle.
Also, I think the commenter was pointing out that it should be interdiCTus/um, like the 4th pp of dicere, rather than interdiXus/um.
Also, you’re right about interdictus. Just a typo on my part; sorry folks.