Nam enjoys going for a picnic at weekends.
(After "enjoys," we use the gerund form of the verb, so it is "going.")

You must not forget.
("Forget" is used in the base form after "must not.")

Where is Mark? - He is helping his mother to cook in the kitchen.
(This is a present continuous tense because the action is happening right now.)

Last night, my children made a lot of noise, so I couldn’t sleep.
(This is the simple past tense because the action happened in the past.)

My father always makes important decisions in my family.
(The present simple tense is used here for a habitual action, as it describes something that happens regularly.)