The suffix -heid turns adjectives into abstract nouns, always feminine (de): vrij (free) → de vrijheid (freedom), mooi (beautiful) → de mooiheid (beauty — though schoonheid is more common), eerlijk (honest) → de eerlijkheid (honesty), veilig (safe) → de veiligheid (safety/security), menselijk (human) → de menselijkheid (humanity). This suffix is one of the most productive in Dutch.
The suffix -ing turns verbs into nouns, often describing a process or result (usually de): oefenen (to practise) → de oefening (exercise/practice), veranderen (to change) → de verandering (change), ontmoeten (to meet) → de ontmoeting (meeting/encounter), vergaderen (to meet formally) → de vergadering (meeting), bouwen (to build) → de bouw/building. Many -ing nouns also have -ing kept from English.
The suffix -er creates agent nouns (who does something): werken (to work) → de werker (worker), leren (to learn/teach) → de leraar/leerling (teacher/learner — irregular), schrijven (to write) → de schrijver (writer), rijden (to drive) → de rijder (rider/driver), fietsen (to cycle) → de fietser (cyclist). The suffix -aar is a variant: aanvaren → de aanvaarder. Understanding these patterns makes encountering new words much less daunting.