Frisian has only one form for the indefinite article, viz. in /ən/. This implies that there is no gender distinction, and that the article is not inflected for case. The article in is only used in combination with singular nouns. Indefiniteness of plurals is marked by the lack of an article or, formulated in different terms, that there is a null article. Hence, the singular indefinite Noun Phrase (NP) in apel an apple has apel-s apple-PL apples as its plural cognate.

Not only does the indefinite pronoun only occur in combination with singular nouns, its selection is even more restricted in that only count nouns are qualified. This restriction is probably a semantic one; the indefinite article, which derives from the numeral ien one historically, picks out one member from a set denoted by the noun. Hence, we have in apel an apple, but not *in wetter a water, since wetter is not a count noun. In addition, the indefinite article does not combine with proper nouns, viz. *in Jan a John. However, in certain special cases these restrictions can be violated, see the section on use below.

The phonological form of the indefinite article is /ən/. This combination of schwa plus dental nasal consonant is open to several phonetic adjustments. The nasal can build a syllable on its own, after deletion of schwa; see syllabic sonorant consonants (read more about subsequent processes). Or the nasal deletes and subsequently the schwa becomes nasalized; a further phonological description may be found in the underlying representation of nasalized vowels: simplex forms. Furthermore, the nasal may become subject to place assimilation.

In cases of nominal ellipsis, the form in turns to ien if the indefinite article is immediately in front of the elided noun. Compare:

Note that it is the form of the numeral ien one that shows up here.

The various uses of the indefinite article will be strictly touched upon in this section. More can be found in the syntactic part of the Taalportaal: function of the indefinite article and the indefinite article before a plural noun in constructions expressing a high degree.

The main function of indefinite NPs is to introduce new information into the discourse domain. Compare:

In certain (intentional) contexts indefinite NPs are ambiguous between a specific or a non-specific reading. In a sentence like

the NP in reade trui can refer to a particular read sweater (specific reading) or any read sweater (non-specific).

Indefinite NPs may also display a generic reading:

The same generic reading can be found in indefinite plurals:

In addition, the indefinite article may occur in unexpected contexts, with a deviating reading as a result. A case in point is the use with a mass noun. This may be allowed, but then the mass noun gets an interpretation as if it were a count noun. In the examples below, we get a "sort" interpretation:

The indefinite article can also be used if the noun refers to a specific quantity of a mass:

In exclamative sentences the indefinite article in can have an expressively qualifying function:

In this way, the indefinite article may also occur in combination with mass nouns and plural nouns. In such exclamative sentences it can be both expressively qualifying and expressively quantifying. Compare:

Normally, the indefinite article is not combined with a proper name. It is allowed, however, to express the meaning a certain, someone who bears the name X:

Something comparable occurs when the indefinite article is placed before a numeral. It then gets a modifying (approximative) function:

This function of in appears to be zero if the quantifying expression has been modified approximatively in another way:

These phrases do not get another meaning if in is omitted, as in lyts tweintich jier etcetera.

We also see the meaning of 'a certain' in a few idiomatic expressions:

We may find the same interpretation with indefinite plurals:

The indefinite article also figures in a typical construction with two temporal expressions. The article belongs to the one which has the longest duration inherently. This NP is preceded by a bare temporal expression. The latter functions as a point in time to which the longer time span is added:

Dutch needs an extra preposition over over in its comparable construction, i.e. vandaag over een week today over a week a week from now. It should be noted that the construction cannot only refer to the future, but also to the past. See the quote from the Frisian writer Reinder Brolsma below:

In referring to the past, Dutch uses the preposition voor for, so Frisian juster in wike yesterday a week ago is gisteren voor een week in Dutch.

Finally, it should be noted that predicative nouns denoting a profession, belief or ideology are not preceded by the indefinite article, in contrast with the English use:

This also holds for predicative nouns following the conjunction as as: