Middle Primitive Elvish
den
root. hole, gap, passage
pen(ed)
root. *hillside, slope
tundu
noun. hole
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 (Noldorin-only?) root in The Etymologies of the 1930s glossed “hole, gap, passage” with the derivative dîn “opening, gap, pass in mountains” (< ✱dēnǝ) which is an element in a couple names from The Etymologies but appearing nowhere else (Ety/DEN).