It is difficult to master Russian pronunciation because the accent is free; that is, it can be placed on any syllable. Thus, there being no set rules for stress, the accent of each word has to be learned separately. In fact, the position of the accent on a given word may vary as the word's case and number change when it is declined. Some words that are spelled alike are distinguished only by a different stress. In addition, no significant differentiation is made between long and short vowels. Grammatically, Russian is highly inflected. The noun has six cases (nominative, genitive, dative, accusative, instrumental, and locative), with an occasional seventh case, the vocative. There are three declensional schemes and three genders, masculine, feminine, and neuter. Although the verb has only three tenses (past, present, and future), it is enabled by a feature called aspect to express numerous subtle shades of meaning, some of which cannot be rendered even in English. In addition the Russian verb has five moods and four voices.