Learn Dutch: Mijn been is lang.

In everyday Dutch, been means leg and bot means bone. However, been can sometimes mean bone in medical contexts or compound words (like sleutelbeen for collarbone).

Why doesn't lang get an -e at the end here?

Adjectives usually only get an -e ending when they come directly in front of a noun (like een lange man). Here, lang comes after the verb is, so it stays in its basic, unchanged form.

What is the plural of been, and why does the spelling change?

The plural is benen (legs). We drop one e because of Dutch spelling rules: when a long vowel sound ends a syllable (be-nen), we only need to write a single vowel letter to keep the long sound.

Since this lesson is about om ... te clauses, how could I add one to this sentence?

You could say something like: Mijn been is te lang om in deze auto te passen (My leg is too long to fit in this car). The word om introduces the purpose, and te is placed immediately before the full verb (passen) at the very end.