Simple Present Tense

THE SIMPLE PRESENT TENSE IS USED:

• To express habits, general truths, repeated actions or unchanging situations, emotions and wishes:
  I smoke (habit); I work in London (unchanging situation); London is a large city (general truth)
• To give instructions or directions:
  You walk for two hundred meters, then you turn left.
• To express fixed arrangements, present or future:
  Your exam starts at 09.00
• To express future time, after some conjunctions: after, when, before, as soon as, until:
  He'll give it to you when you come next Saturday.

FORMING THE SIMPLE PRESENT TENSE: TO THINK

Affirmative	Interrogative	Negative
I think	Do I think?	I do not think
You think	Do you think?	You do not think
He thinks	Does he think?	He does not think
She thinks	Does she think?	She does not think
It thinks	Does it think?	It does not think
We think	Do we think?	We do not think.
They think	Do they think?	They do not think.

NOTES ON THE SIMPLE PRESENT, THIRD PERSON SINGULAR

• In the third person singular the verb always ends in -s:
  he wants, she needs, he gives, she thinks.
• Negative and question forms use DOES (= the third person of the auxiliary 'DO') + the infinitive of the verb.
  He wants ice cream. Does he want strawberry? He does not want vanilla.
• Verbs ending in -y : the third person changes the -y to -ies:
  fly --> flies, cry --> cries
  Exception: if there is a vowel before the -y:
  play --> plays, pray --> prays
• Add -es to verbs ending in:-ss, -x, -sh, -ch:
  he passes, she catches, he fixes, it pushes

reference: http://www.ef.com/english-resources/english-grammar/simple-present-tense/