Looking in the dictionary for the adjective boring, I get...
> boring
> enuiga, enua, teda

If I want to say...
- Politics is boring.
- What a boring day!
- Boring! (not sure what part of grammar this belongs).
- The boring speech lasted 1 hour.

Which should I use?

Thanks.

According to ReVo, "enui" means "Malplezuri pro senintereso aŭ neokupiteco" (to be displeased out of lack of interest or occupation), while "[/quote]tedi" means "Kaŭzi lacecon kaj malagrablecon pro trodaŭro, troripetiĝo aŭ troabundo" (to tire or annoy out of repetition, duration or excess).

Remember that 'enui' is intransitive. If I am boring you, then "mi enuigas vin".

"Mi enuas vin" means, instead, "I am bored because of you".

So, I would say:

Politiko tedas.
Kia enuiga tago!
Tede!
La teda prelego/parolado daŭris unu horon.