1. He knifed a person last night.

2. He knives a person every year.
3. He knifes a person every year.
4. There are three knives on the table.

Which one is incorrect among the four?
Would you check the conjugation of the verb knife or the noun knife?

#2 is not correct; #3 is correct, as are #s 1 and 4.

VERB:
to knife (to stab with a knife)

knife = present form with knifes when the subject is 3rd person singular

knifed = simple past form

knifed = past participle form (used with "to be" and "to have" auxiliary verbs)

NOUN:
knife = singular
knives = plural
knife's = singular possessive
knifes' = plural possessive

Sorry. That last noun form should be knives' -- since it's plural!

The incorrect sentence is:

2. He knives a person every year.

The correct sentence should be:

2. He knifes a person every year.

When determining the correctness of the sentences, we need to check the verb conjugation, not the noun knife.

In this case, the verb "knife" is irregular in its conjugation. In the present tense, the third-person singular form (he/she/it) should be "knifes" rather than "knives." Therefore, sentence 2 is grammatically incorrect.