-sum mainly attaches to stems of verbs. An exception is betochtsumcautious, deliberate which has a past participle (the infinitive of this strong verb is betinkethink (about), consider) as its base.

Typical for the suffix -sum is the modal-causative reading in derivations like leareto learn > learsuminformative, droegjeto dry > droechsumdrying, groeieto grow > groeisumgrowing and wurkjeto work > wurksumworking.

A modal-passive reading "possible to be {verb}-ed" can be found in brûketo use > brûksumusable and sjongeto sing > sjongsumsingable. So in sjongsum ferske is a song that can be easily sung.

An active modality reading "having ability to {verb}" can be found in: strekketo extend > streksumextendable, duorjeto last > duorsumdurable, bekommeto recover > bekomsumrecoverable, rêdeto save > rêdsumpossible to save and ferfeleto be bored > ferfeelsumuncomfortable.

An necessity reading can be found in grouweto be horrified > grousumhorrible. So, grousum waar is weather one has to be horrified of.

A non-modal reading "having tendency to {verb}" can be found in skreppeto work hard > skrepsumdiligent, behelpeto manage > behelpsumhelpful, betrouweto trust > betrousumreliable and fertelleto tell > fertelsumcommunicative.

There are only a few adjectives ending in -sum that have a noun as base, mostly denoting some abstract virtue. Hence, the derived adjectives have a positive connotation. Examples are deugdvirtue > deugdsumvirtuous, earrespect > earsumrespectable, hânhand > hânsumhandy, manageable and fredepeace > freedsumpeaceful.

An adjectival base is rare. Examples are gemiencommon > gemiensumcommon, shared and seldenrarely > seldsumrare. The latter case shows truncation of -en.

The suffix -sum can also have a numeral as its base form. There is only one numeral to which it is attached: ienone, iensumlonely. Other numerals + -sum are ungrammatical: twatwo > *twasum, fjouwereight > *fjouwersum. The same holds for numeral+-lik, which can also only be added to ienone.

-sum[səm] is a non-cohering suffix and does not bear stress. However, it can influence the stress pattern of the base as it requires the main stress to be on the last stem syllable. This is relevant for particle verbs, for example in OPmerkeremark > opMERKsumremarkable.

If a base word ends in a schwa, this schwa is truncated: fredepeace > freedsumpeaceful. We see truncation of a schwa syllable -en in seldenrarely > seldsumrare.

Adjectives ending in -sum can serve as input for other words, especially nominalizations in -heid (seldsumheidrarity) and adjectives prefixed with ûn- (ûnbrûksumuseless). In some cases, only the ûn- form exists: ûnachtsumcareless but *achtsum. Verbalization is not possible, with the exception of ferûnachtsumjeto neglect and feriensumjeto grow lonely.