Third person singular -s

The short answer is that, except for modal verbs, the third person singular in the simple present tense always ends in –s: she climbs, he runs, it rains, etc. 

Now for a more detailed answer: For the vast majority of verbs, the third person singular in the simple present is formed by adding –s to the main form. However there are a few spelling rules and irregular verbs to be aware of. 

Spelling rules

Add –es instead of –s if the base form ends in -s, -z, -x, -sh, -ch, or the vowel o (but not -oo). This adds an extra syllable to the word in spoken form. 

• miss + es = misses         He misses her so much.
• veto + es =  vetoes         She vetoes every idea that I suggest.

If the base form ends in consonant + y, remove the -y and add –ies:

• rely --> relies                 Tom relies on her. 
• worry --> worries           My father worries about me.

Irregular verbs

Two very common irregular verbs that you already know do not follow the rules above (although their third person singular present forms do actually end in –s):

• be --> is
• have --> has

Modals

Finally, as mentioned above, the modal verbs, such as can, must, should, may and might, do not take -s in the third person singular present because, as you probably know, modal verbs do not take endings at all. 

• She can speak three languages.
• He must like football a lot.