Subject Word Agreement Confusion

Question

I was typing something, and found my grammer tool correcting me on the following sentence –

They makes a nice couple.

The correct sentence suggested was –

They make a nice couple.

When googled more on it I found that – Subjects and verbs must agree in number. If the subject is singular, the verb must be singular too. And if the subject is plural, the verb must also be plural.

Now that added more to my confusion, I thought verbs ending with “s” should be considered plural and not ending with “s” should be singular.

Some more examples –

Incorrect: She write every day.

Correct: She writes every day.

Incorrect: They writes every day.

Correct: They write every day.

Incorrect: He fix the car when it break down.

Correct: He fixes the car when it breaks down.

And then, why I love you is correct and not I loves you is incorrect, when "I" is also singular, as "He" and "She"?

Answer 1

Now that added more to my confusion, I thought verbs ending with “s” should be considered plural and not ending with “s” should be singular.

That is how nouns work. In English, many nouns can be made plural by adding "s" to the end, but verbs are the opposite. You make verbs plural by removing the ending "s". This principle is best illustrated with examples where the "s" is "moved" from the verb to the noun when going from singular to plural, like so.

Singular:

The cow chews the cud.

Plural:

The cows chew the cud.

Note the "s" has "moved" from the verb "chew" to the noun "cow". Granted this will not always work because plural forms can vary, but it may help for remembering the rule starting out.

Answer 2

I think I know where your confusion comes from. The s that you think it's coming from the plural form of nouns it's not actually a sign for plural but the way you are forming the present tense simple.

So the s in horses is not the same s as in he thinks.

Present tense simple has a special form for the 3rd person singular (he/she/it) by adding an s to the form of the verb.

She thinks she knows the truth.

The sun rises from the east.

Hope this solves your confusion.

Answer 3

OP has two confusions in Subject Verb Agreement -

1. singular plural verbs and
2. verbs with first person singular.

I. Singular and plural verbs

When googled more on it I found that -

Subjects and verbs must agree in number. If the subject is singular, the verb must be singular too. And if the subject is plural, the verb must also be plural.

In present tenses, nouns and verbs form plurals in opposite ways:
-nouns ADD an 's' to the singular form.
-but verbs REMOVE an 's' from the singular form.
Ref.: towson.edu https://webapps.towson.edu/ows/sub-verb.htm

II. Third person singular verbs in present tense.

And then, why I love you is correct and not I loves you is incorrect, when "I" is also singular, as "He" and "She"?

Singular verbs are applied only for third person singular subject in the present

Verbs are conjugated depending on the subject. For regular verbs, we add an -s or -es to the third-person singular form. We typically only add -es to verbs ending in -s, -x, -z, -ch, or -sh. For the rest of the forms, we add nothing.
Examples:
I sit. You sit. He sits. We sit. They sit.
I come. You come. She comes. You and I come. The dogs come.
I watch. You watch. The child watches. We watch. They watch.
Ref.: writingaid.com https://prowritingaid.com/grammar/1000139/Subject-Verb-Agreement