Prepositions and adverbs can have a literal meaning which is spatial or "orientational", and then, as happens with all words, metaphorical meanings develop that are systematic extensions from the original core meaning.[14] Many verbs in English can interact with an adverb or a preposition, and the verb + preposition/adverb complex is readily understood when used in its literal sense. He walked across the square. She opened the shutters and looked outside. When he heard the crash, he looked up. The function of the prepositional phrase/particle in such clauses is to show the relationship between the action (walked, opened, looked) and the relative positioning, action or state of the subject. Even when such prepositions appear alone and are hence adverbs/particles, they have a retrievable prepositional object. Thus, He walked across clearly shows that the "walking" is "across" a given area. In the case of He walked across the square, across the square is a prepositional phrase (with across as its head word). In both cases, the single-word/multi-word expression (across and across the square) is independent of the verb. The action of the subject (walking) is being portrayed as having happened in/at/on/over a certain location (across the square). Similarly in She opened the shutters and looked outside and When he heard the crash, he looked up, outside is logically outside (of) the house, and up is similarly an adjunct (= upwards, in an upwards direction, he is looking in a direction that is higher than where his eyes were previously directed). An object in grammar is part of a sentence, and often part of the predicate. It denotes somebody or something involved in the subject's "performance" of the verb. Basically, it is what or whom the verb is acting upon. As an example, the following sentence is given: In the sentence "Bobby scored a goal", "a goal" is the object. "Bobby" is the subject (the agent, doer, or performer of the action), "score" is the action, and "goal" is the object (what or whom the action of the verb is acting upon). The verb in the clause determines whether there can or must be objects in the sentence, and if so how many and of what type. (See also Valency (linguistics).) In many languages, including English, the same verb can allow different structures: "Bobby scored" and "Bobby scored a goal" are both valid English sentences. Note that the meaning of the verb can be affected by the presence or absence of an object. Objects fall into classes: direct objects, adpositional objects, and non-prepositional indirect objects. A direct object answers the question "What?", while an indirect object answers the question "To whom?" or "For whom?". An indirect object is the recipient of the direct object, or an otherwise affected participant in the event. There must be a direct object for an indirect object to be placed in a sentence. Some examples: In "The girl ate fruit", fruit is a direct object of the verb ate. It corresponds to the accusative of languages with grammatical cases. In "They sent him a postcard", him is a (non-prepositional) indirect object of the verb sent (which uses a double-object construction). It typically corresponds to the dative case. In "I envied him his success", his success is an oblique object of the verb envied (it could be expressed for his success instead). In "We listened to the radio", radio is the object of the preposition to, and the prepositional object of the simple past of the phrasal verb to listen to. It can correspond to a variety of cases and complements. In many languages, including German, Latin, and Classical Arabic, objects can change form slightly (decline) to indicate what kind of object they are (their case). This does not happen in English (except for a few pronouns that do have separate subject and object forms, such as he versus him); rather, the type of object is indicated strictly by word order. Also, some objects are treated differently from others in particular languages. In Spanish, for example, human objects have to get a preposition 'a'. This is called differential object marking.