Irregular affirmative tú commands are NOT like the present tense él form of the verb. Let's look at some irregular verbs.
decir - di! - Say!, Tell!
Di cuándo vienes.
Tell (say) when you come.
poner - Pon! - Put!
Pon tres cucharadas de aceite en una sartén.
Put three spoonfuls of oil into a frying-pan.
salir - Sal! - Leave!
Sal y compra el billete.
Leave (go) and buy the ticket.
tener - Ten! - Have!
Ten cuidado, muchacha.
Be careful, girl.
venir - Ven! - Come!
ir - Ve! - Go!
Ve al garaje.
Go to the garage.
Because the tú command for ser is the same as the present tense yo form of the verb saber, you must consider the context of the sentence in order to determine which of the two is intended. In the following examples, notice how using a subject pronoun clarifies these identical forms with completely different meanings.
Sé tú médico porque así ganarás mucho dinero.
Be a doctor because you will earn a lot of money. (affirmative tú command, ser)
Yo sé que los médicos ganan mucho.
I know that doctors earn a lot. (present tense yo form, saber)
If you tell someone whom you address as tú not to do something, it is a negative command and, thus, you must use a different form. Only the tú and the vosotros commands have different forms for the negative and the affirmative. For Ud. and Uds. commands, the same form is used for negative and affirmative commands.
Irregular affirmative tú commands
There are a few affirmative tú commands that are not like the present tense él form of the verb; these are considered irregular. The table below shows the the irregular affirmative tú commands for seven verbs.
Infinitive Affirmative Tú Command Translation
tener ten have
poner pon put
salir sal leave
venir ven come
hacer haz make or do
decir di say or tell
ser sé be
ir ve go