Yo siempre digo la verdad.
You can use any form of the verb, but it has to be indicative. I have used the <u>present form</u> (the most used in these cases) of the verb "decir" (say).
<h3><em><u>MissSpanish</u></em></h3>
Answer:
El Everest.
Explanation:
El Everest es la montaña más grande del mundo.
...