The form of the verb varies with person (first, second and third), number (singular and plural), tense (present and past), and mood (indicative, subjunctive and imperative). Old English also sometimes uses compound constructions to express other verbal aspects, the future and the passive voice; in these we see the beginnings of the compound tenses of Modern English. Old English verbs include strong verbs, which form the past tense by altering the root vowel, and weak verbs, which use a suffix such as -de. As in Modern English, and peculiar to the Germanic languages, the verbs formed two great classes: weak (regular), and strong (irregular). Like today, Old English had fewer strong verbs, and many of these have over time decayed into weak forms. Then, as now, dental suffixes indicated the past tense of the weak verbs, as in work and worked.