The abstract suffix is one of the most useful tools I have found for creating non attested words from already existing Sindarin words. It can be used on nouns, adjectives and verbs, and I shall attempt to illustrate the point of it below with some attested examples.

Sindarin word	With the abstract suffix becomes
Aeg (point)	Aegas (mountain peak)
Bain (beautiful)	Bainas (beauty)
Gala- (to grow)	Galas (growth, a plant)
Ovor (abundant)	Ovras (crowd, heap)
Pent (tale)	Pennas (history)
Rîn (remembrance)	Rínas (memory)

Once you get the hang of using it, the abstract suffix is a powerful tool to help you with translations.
Adjectives and verbs are relatively straight forward - take an adjective such as hand = intelligent, and it becomes hannas = intelligence, understanding.
Nouns are a little less easy to treat. The abstract version of a noun becomes a greater version of the original, for example: hûb = a harbour becomes hobas = harbourage or ind = inner thought, meaning becomes innas = will.

Creating words with the abstract suffix

The abstract suffix is -as. The rules for using it are the same as for the suffix -ath (change the last letter if you need to before attaching the abstract suffix). You can find the rules here.

Examples:
Balchas (cruelty) from Balch (cruel)
Berenas (bravery, courage, boldness) from Beren (brave, bold)
Faelas (justice, fairness) from Fael (fair-minded, just)
Gadas (a trap) from Gad- ('to catch')