A preposition is a word that links two parts of a sentence, showing semantic relations such as place, time, cause, or accompaniment. In Portuguese, the preposition does not change and usually comes before a noun, pronoun, or verb in the infinitive. Using the right preposition makes the sentence clear and natural. The most common prepositions appear in many everyday contexts.

"De" indicates origin, possession, material, or the theme of something. It also appears in fixed expressions and with verbs that require "de". Often, "de" joins with articles and pronouns forming contractions. The exact meaning depends on the context of the sentence.

"Em" indicates location, defined time, and state or situation. It is the basic preposition to say where something is or happens. With definite articles, "em" forms contractions such as "no" and "na". The use of "em" is different from "a" when we talk about destination.

"A" indicates destination, direction, and scheduled time. In sentences with movement, 'a' shows where someone is going. With hours, we use 'a' to mark the moment. Be careful not to confuse with the feminine article 'a'.

"Com" indicates companionship, instrument, and manner. We use "with" to say who we are with or with what we do something. It can also appear in expressions of manner. "With" does not contract with articles, but can come before pronouns.

"Para" indicates purpose, destination, and recipient. We use "para" when we want to state the purpose of an action. It also appears to mark the direction of a movement, especially in Brazilian Portuguese. "Para" can be reduced to "pra" in informal speech, but the sentence structure does not change.

"Por" indicates cause, reason, exchange, means, and approximate duration. We use "por" to explain the reason for an action and to talk about means such as transport or communication. It also appears in expressions of non-exact time. "Por" contracts with articles, forming words like "pelo" and "pela".

In Portuguese, prepositions join with articles and pronouns to form contractions. The contractions preserve the meaning of the preposition and make the sentence more natural. The forms "do", "da", "no" and "na" are very common with places and objects. Using the correct contraction depends on the gender and number of the noun.

"De" usually marks origin or possession, "por" marks reason or means, and "para" marks purpose or destination. The central difference is the sense the sentence wants to convey. In many cases, a verb or expression already requires a specific preposition. Comparing the examples helps to fix the correct usage.

With place names, the preposition may change depending on the idea of being there or going. 'Em' usually indicates permanence or location, and 'a' indicates movement as a destination. The contractions 'no' and 'na' appear with many places that have an article. Some city and country names are used without an article, so the preposition remains uncontracted.

The use of "para" as destination is more common in Brazilian Portuguese, especially in speech. In Portugal, "a" appears more with destinations formal and with means of public transportation. The reduction "pra" is typical of Brazil in informal speech. The overall meaning does not change, but the choice sounds more natural in each variant.