The structure of language is referred to as form. This contains the sounds that make up words, little parts of words that add meaning (for example, a "s" at the end of a word to make it signify "more than one"), and sentence grammatical structures and rules. Language learners must be aware of these elements if they are to have any chance of being successful at communicating their ideas and opinions. In general, languages tend to share certain forms. For example, most languages have regular noun inflection-that is, they change the form of the word to reflect its role in the sentence: singular vs. plural, human vs. animal, etc.-and there are also common patterns for verbs, pronouns, adjectives, and adverbs. Knowing these regular changes can help language learners understand how words in a new language are used. Some forms are more important than others, though. For example, English lacks a simple way to indicate the number of objects being referred to by a single term (such as "two dogs"). Instead, speakers must use multiple words or phrases to convey this information. Other forms such as human vs. animal subjects are highly significant but usually invisible to speakers of English. Even so, many form differences between languages are due to social factors rather than semantic ones. For example, Latin has no simple way to indicate a direct object; instead, it requires an additional word like "for" or "to".