Early Primitive Elvish
teŋe
root. know, understand
aha
root. know
ŋolo
root. to know
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A root in the Qenya Lexicon of the 1910s glossed “know”, but most of its derivatives have to do with “mind” (QL/29). There are quite a few later roots filling this same semantic space, and this root was probably abandoned.