Decide if the following commands is grammatically correct of incorrect.

(tu) no digo mentiras.
incorrect
correct

(tu) no escribas cosas tontas.
incorrect
correct

For this case we have to:

"Yo no digo mentiras (I don't tell lies)"   is the correct form of the first sentence.

The present indicative of the verb "decir" we have:

Yo digo

tú dices

él/ella/Ud. dice

nosotros decimos

vosotros decís

ellos/ellas/Uds. dicen

Thus, the first option is incorrect.

On the other hand we have:

"tú no escribas cosas tontas" is correct, generally used "do not write silly things (no escribas cosas tontas)" knowing that it refers to the second singular person.

From the present subjunctive of the verb "escribir" we have:

Yo escriba

tú escribas

él/ella/Ud. escriba

nosotros escribamos

vosotros escribáis

ellos/ellas/Uds. escriban

Option A: Incorrect

Option B: Correct