"Ich mag Wasser."

Translation: I like water.

[maryyphelan]

mag is a part of a verb called mögen which is a modal verb. There are 6 modal verbs and they are all irregular.

[Au_Dup]

Ich mag Bier

[calypso15]

When conjugating "mögen", why is it "er/sie/es mag" rather than "er/sie/es magt"?

[AgenTsi]

It's an irregular verb. The conjugations are different from what you'd normally expect.

[Alejandro.573992]

Because it is an irregular verb