Finger tips to add "es" and "s" to an action verb to convert a base form into a third person singular form

Question

Please let me know, when we should add es or s to an action verb to convert a base form into a third person singular form? I meant to ask are there any finger tips we can follow?

Examples:

go = goes

Walk = walks

Answer 1

Keep in mind that there are both regular and irregular verbs. If we just go by the examples you've provided, we could hypothesize that the "s" ending follows a (regular) verb root that ends with a consonant and "es" goes with a (regular) verb root that ends with a vowel--usually an 'o'. Verbs that end with 'y,' like "to fly" become "flies." Verbs that already end with an "e" just need an 's' to conjugate in the third person. I'd say that those are decent rules of thumb.

Answer 2

Obfuskater gave sound rules of thumb. Here are slightly fuller guidelines:

For the third person singular we use the –s form of the verb: Luke seems happy.

Spelling of the –s form

-e is added before –s in verbs ending in a sibilant sound (represented by the letters -s, –ss, -z, -sh, -ch and –x) and in a single –o (but not those ending in –oo):

hiss/hisses, buzz/buzzes, wash/washes, catch/catches, box/boxes, go/goes
but: boo/boos

In verbs with word-final single –s, some writers double the final –s of the first form before adding the –e; this is optional:

gas/gases or *gasses, focus/focuses or focuses

With verbs ending in –y following a consonant, we change the –y to –i and add -es, but if the word-final –y follows a vowel/diphthong sound, we simply add –s:

carry/carries, try/tries but: play/plays, employ/employs

Pronunciation

The final –(e)s of this form is pronounced:

/s/ after unvoiced consonants (except /s, ʃ, ʧ/: /kɪks, pʊts, kɒfs/).

/z/ after voiced consonants (except /z, ʒ, ʤ/), vowels and diphthongs: /hʌgz, siːz, leɪz/

/ɪz/(or/əz/) after /s, z, ʃ, ʒ, ʧ/ and /ʤ/: /bɒksɪz, bʌzɪz, wɒʃɪz ruːʒɪz, kæʧɪz, ʤʌʤɪz/

Exceptions

One verb, BE. is completely irregular in the unmarked tense: I am, he is, we/you/they are.

One verb, HAVE. has an irregular third person singular form: /hæz/.

Two verbs, DO and SAY, have pronunciation changes in the third person singular form: do, does /duː, dʌz/, say, says /seɪ, sez/

Answer 3

Irregular verbs — those that end with x, z, ch, sh, ss and o — we can add es. Verbs ending with y but followed by a consonant, we should remove y and add ies.