Frisian has two infinitives, one ending in -e and and the other ending in -en. In the part on verbal inflection these are dubbed infinitive I and infinitive II. The infinitives obey a strict distribution, depending on syntactic context, as is described in the section infinitive of the topic on general categories of verbal inflection. Monosyllabic verbs like sjento see or geanto go are the exception: their infinitive invariably ends in -n. For monosyllabic verbs, see strong inflection - monosyllabic verbs.

If a verb is transposed to a noun, which in the unmarked case is an action noun, it always has the ending -en. The gender of the noun is neuter, as can be detected from the selection of the neuter definite article it or the neuter demonstrative pronoun dat. Hence, we have it rinnenthe walking or dat ridenthat skating. As can be seen from these examples, the occurrence of -en is independent of the existence of the two weak inflectional classes in Frisian: it can be attached to verbs that have their infinitive I ending in -e and in -je alike. The nominalized infinitive can occur in various syntactic configurations; see complementation of infinitival Noun Phrases (NPs) for an impression.

A difference from Dutch is the fact that Frisian shows more semantic specialization in these nominal infinitives. Possibly, this is an effect of the circumstance that the language has this special infinitive -en for nominal contexts, and/or this suffix is used quite frequently for such contexts, at the cost of other suffixes, like -ing. To illustrate this, Frisian and Dutch are put side by side in this section. The first examples are:

In the corresponding examples, Dutch rather uses a noun or a different verbal description.

Frisian also allows for the nominalisations dwaandoing, sizzensaying and kommencoming, while in Dutch a different expression has to be used. Examples are:

Furthermore, in a number of cases the suffix -en can be used to form object nouns. The most important are:

Infinitive I	Infinitive II as object noun
iteto eat	itenfood
drinketo drink	drinkendrink(s)
frettento feed	frettenfodder
struieto scatter	struienlitter
lêzeto read	lêzenreading matter
naaieto sew	naaienneedlework
breidzjeto knit	breidzjenknitting
striketo iron	strikenclothes for ironing; ironed clothes
ôfwaskjeto wash the dishes	ôfwaskjendishes

In such cases, Dutch usually takes recourse to the usual instruments of word formation, like compounding or conversion:

Finally, Frisian nominal infinitives with -en may be used in a construction with the preposition opon:

Again, Dutch uses full nouns here, as in op zichtfor your perusal or op afbetalingon hire purchase.

The suffix is basically pronounced as [ən]. Mostly, however, /n/ becomes syllabic, after deletion of the schwa (more information in syllabic sonorant consonants). As the suffix is pronounced with a schwa, it bears no stress, and suffixation with -en does not alter the stress pattern of the base.

Nouns in -en do not allow a plural: skriuwenwriting > *skriuwenswriting-PL). An exception is the lexicalized form oantinkensouvenir > oantinkenssouvenirs. Nor can the derivations constitute the first part of a compound: ôfbeteljenpart-paying > *ôfbeteljensysteempart-paying-systemhire purchase system. However, lexicalized exceptions exist, as in frisdrinkensflessesoft-drink-LINK-bottlesoft drink bottle, with a linking element -s-.