Third person -s
The changes with the verb be are rather complicated. The present tense indicative third person singular form is "is" ("he is", "she is" "it is" "one is"), containing this suffix. Most other forms are "are" (second person singular and plural "you are", first person plural "we are", third person plural "they are"). In the indicative past tense, this suffix alters "was" to "were" but only for the first person singular ("I was") and third person singular ("he was", "she was", "it was", "one was").
The verb have has a small pronunciation and spelling change - when third person -s is added, /v/ or -ve- is removed, so we say "it has" not *"it haves".
Although present in standard English, it is not found in some non-standard varieties.
Although it is a relatively straightforward rule, there is often significant delay between students first encountering it and their being able to reliably produce it.