perfect), and the old prefix-conjugated non-past (or imperfect) with gemination was discarded. New suffixes were used to mark different moods in the non-past, e.g. Classical Arabic -u (indicative), -a (subjunctive), vs no suffix (jussive). (It is not generally agreed whether the systems of the various Semitic languages are better interpreted in terms of tense, i.e. past vs. non-past, or aspect, i.e. perfect vs. imperfect.) A special feature in classical Hebrew is the waw-consecutive, prefixing a verb form with the letter waw in order to change its tense or aspect. The South Semitic languages show a system somewhere between the East and Central Semitic languages. Later languages show further developments. In the modern varieties of Arabic, for example, the old mood suffixes were dropped, and new mood prefixes developed (e.g. bi- for indicative vs. no prefix for subjunctive in many varieties). In the extreme case of Neo-Aramaic, the verb conjugations have been entirely reworked under Iranian influence. Morphology: triliteral roots All Semitic languages exhibit a unique pattern of stems called Semitic roots consisting typically of triliteral, or three-consonant consonantal roots (two- and four-consonant roots also exist), from which nouns, adjectives, and verbs are formed in various ways (e.g., by inserting vowels, doubling consonants, lengthening vowels or by adding prefixes, suffixes, or infixes). For instance, the root k-t-b, (dealing with "writing" generally) yields in Arabic: katabtu كَتَبْتُ or كتبت "I wrote" (f and m) yuktab(u) يُكْتَبُ or يكتب "being written" (masculine) tuktab(u) تُكتَبُ or تكتب "being written" (feminine) yatakātabūn(a) يَتَكَاتَبُونَ or يتكاتبون "they write to each other" (masculine) istiktāb اِسْتِكْتابَ or استكتاب "causing to write" kitāb كِتابٌ or كتاب "book" (the hyphen shows end of stem before various case endings) kutayyib كُتَيِّبٌ or كتيب "booklet" (diminutive) kitābah كِتابةٌ or كتابة "writing" kuttāb كُتّابٌ or كتاب "writers" (broken plural) katabah كَتَبةٌ or كتبة "clerks" (broken plural) maktab مَكْتَبٌ or مكتب "desk" or "office" maktabah مَكْتَبةٌ or مكتبة "library" or "bookshop" maktūb مَكْتُوبٌ or مكتوب "written" (participle) or "postal letter" (noun) katībah كَتِيبةٌ or كتيبة "squadron" or "document" iktitāb اِكْتِتابٌ or اكتتاب "registration" or "contribution of funds" muktatib مُكتَتِبٌ or مكتتب "subscription" and the same root in Hebrew: (A line under k and b mean a fricative, x for k and v for b.) kāṯaḇti כָּתַבְתִּי or כתבתי "I wrote" kattāḇ כַּתָּב or כתב "reporter" (m) katteḇeṯ כַּתָּבֶת or כתבת "reporter" (f) kattāḇā כַּתָּבָה or כתבה "article" miḵtāḇ מִכְתָּב or מכתב "postal letter" miḵtāḇā