Common words:
* am
* at
* on
* in
* up
* to
* is
* it
* if
* as
* so
* go
* no
* do
* by
* of
Less common words:
* ax (short for "axe")
* ox
* oh
* he
* me
* we
* us
* my
* be
* an
* or
* a
* I
There are also many two-letter abbreviations and symbols that are used as words, like:
* OK
* TV
* CD
* AD
* ID
* AS
* & (and)
So, while there are some rules about what makes a "word," two-letter words are definitely a part of the English language!