A prepositional phrase is a fixed group of two or more words that together function as a preposition. It usually starts with a core such as 'de', 'a', 'em' or 'por' and ends with a noun or an adjective. Between the start and the end, determiners or small prepositions may appear. The prepositional phrase requires the same type of complement as a simple preposition.

The most common prepositional phrases follow the pattern 'preposition + noun + de' or 'preposition + noun'. The final element tends to be fixed and does not vary in number or gender within the phrase. The meaning of the sentence depends on the whole phrase, not the individual words. Many phrases alternate with simple prepositions in some contexts, but may shift emphasis or clarity.

Some locutions indicate place or position and answer questions such as 'where' or 'to where'. They typically appear with verbs of staying, movement, or location. The complement of the locution can be a noun with an article or a pronoun. The clarity of the place depends on using the complete locution without omitting words.

Prepositional phrases also mark time, duration, or anteriority. They typically appear with events, dates, or intervals. The complement is usually a temporal noun or a time expression. In many cases, the phrase avoids ambiguity that could arise with a simple preposition.

Some locutions introduce motive, reason, or objective and are frequently linked to abstract nouns. They appear often in formal texts, explanations, and instructions. The expected complement is normally a noun or a reduced clause with a verb in the infinitive. Using the locution precisely avoids misinterpretations about cause or intention.

Locutions of conformity express agreement with rules, standards, or models. They appear frequently in institutional, legal, or academic contexts. The typical complement is a noun naming the rule or authority. They often equate, in sense, to 'according to', but with a more formal or explicit tone.

Some locutions introduce the topic about which one speaks, writes, or thinks. They appear with verbs such as 'to speak', 'to deal with', 'to think', and 'to write'. The complement is the subject, usually with a definite article. In many cases, the locution avoids constructions like 'about of'.

Most prepositional phrases require a complement with a definite article or another determiner. This happens because the nominal head of the phrase functions as a specific noun. In standard Portuguese, saying 'em cima mesa' or 'por causa chuva' sounds incomplete. The determiner is part of the phrase's fluency in real use.

Prepositional phrases have a fixed word order and do not permit insertions in the middle. Adjectives and adverbs do not enter the phrase without breaking the unit. Variation occurs in the complement, not in the phrase itself. In formal texts, preserving the fixed form avoids ambiguities.

Prepositional phrases are fixed expressions that function as prepositions and introduce a complement. The most frequent express place, time, cause, conformity, and reference. The correct usage depends on choosing the entire locution and completing it with the appropriate determiner. Recognizing the structure helps avoid omissions and inappropriate substitutions. Practicing with complete sentences consolidates the pattern of Portuguese.