Learn Dutch: Het kapotte apparaat is gelukkig repareerbaar. - The broken appliance is fortunately repairable.

To form an adjective expressing potentiality (meaning something 'can be' done), you add the suffix -baar to the verb stem. The stem of repareren (to repair) is repareer, so we get repareerbaar (repairable). This works just like the English suffix '-able'.

Why does repareerbaar have a double 'e' (ee) when the infinitive repareren only has a single 'e'?

This is due to standard Dutch spelling rules for open and closed syllables. In repareren, the second 'e' is at the end of an open syllable (re-pa-re-ren), so it is a long vowel written with one letter. To find the stem, we drop the -en, leaving a closed syllable. To keep that same long 'e' sound, we must double the vowel to make repareer. Then we just add -baar.

Both kapotte and repareerbaar describe het apparaat, so why does only kapotte get an extra -e at the end?

It depends on the position of the adjective. Kapotte is an attributive adjective (placed directly in front of the noun), so it gets an -e because it follows the definite article het. However, repareerbaar is a predicative adjective (it comes after the linking verb is). Predicative adjectives never get the -e ending.