What are the five most common words in the English language?
This question is not as simple as it sounds. Even professional linguists disagree about what such a list would be, and whether it is even possible to make a valid one.
The main problem is that it is impossible to study all uses of the English language, so any answer provided will depend on the sample of language studied. People speak different words than they write, and vocabulary varies with the region or country of the speaker, the speaker's age and education level and other factors.
Some people have taken a stab at it nonetheless. For example, according to the Brown Corpus, [W. Nelson Francis and H. Kucera, Frequency Analysis of English Usage: Lexicon and Grammar (Houghton Mifflin, 1982): p. 465.], these are the top ten most frequently used words in English, with their part of speech:
and (coordinating conjunction)
he (personal pronoun)
to (infinitive mark)
(Note: this list includes base forms only, so the inclusion of the verb "be" implies all the conjugated forms of the verb: "is," "are," "am," etc.)
Online, there is this list (in frequency order) of the 100 most commonly used words in written English.