Icelandic retains many grammatical features of other ancient Germanic languages, and resembles Old Norwegian before its inflection was greatly simplified. Modern Icelandic is still a heavily inflected language with four cases: nominative, accusative, dative and genitive. Icelandic nouns can have one of three grammatical genders —masculine, feminine or neuter. There are two main declension paradigms for each gender: strong and weak nouns, which are furthermore divided in smaller paradigms for declension, according to many criteria (sound-shifts, consonant clusters etc.) Nouns, adjectives and pronouns are declined in the four cases, and for number in the singular and plural. T-V distinction ("þérun") in modern Icelandic seems on the verge of extinction, yet can still be found, especially in structured official address. Verbs are conjugated for tense, mood, person, number and voice. There are three voices: active, passive and middle (or medial); but it may be debated whether the middle voice is a voice or simply an independent class of verbs of its own.[vague] They have up to ten tenses, but Icelandic, like English, forms most of these with auxiliary verbs. There are three main groups of verbs in Icelandic: -a, -i, and -ur, referring to the endings that these verbs take when conjugated in the first person singular present. Some Icelandic infinitives end with the -ja suffix. For many verbs that require an object, a reflexive pronoun can be used instead. The case of the pronoun depends on the case that the verb governs. The basic word order in Icelandic is subject-verb-object. However, as words are heavily inflected, the word order is fairly flexible.
Íslenska hefur haldið mörgum málfræðilegum eiginleikum frá öðrum fornum germönskum tungumálum, og líkist fornnorsku eins og hún var áður en hún missti beygingaleika sinn. Nútíma íslenska er en töluvert beygð og inniheldur fjögur föll: nefnifall, þolfall, þágufall og eignarfall. Íslensk nafnorð geta tekið eitt af föllunum þremur - karlkyn, kvenkyn eða hvorugkyn. Fyrir hvert kyn eru tvö beygingardæmi: sterk og veik nafnorð, sem svo skiptast í fleiri beygingardæmi eftir fallbeygingu, sem ræðst af mörgum viðmiðum (hljóðvörpum, samhljóðaþyrpingu o.fl.). Nafnorð, lýsingarorð og fornöfn fallbeygjast í fjórum föllum, einnig í eintölu og fleirtölu.