em + um = num

The word num is a fundamental contraction in Portuguese. It is formed by combining the preposition em with the indefinite article um.

1. The Full Set:
While 'num' is the most common, the contraction changes based on gender and number:

• em + um = num (masculine singular)
• em + uma = numa (feminine singular)
• em + uns = nuns (masculine plural)
• em + umas = numas (feminine plural)

2. Usage:
It is used to say 'in a', 'on a', or 'at a'. It indicates something non-specific.

• Moro num apartamento. (I live in an apartment — any apartment).

Additionally, num (or its variations) is used when a Portuguese verb requires the preposition em (such as pensar em or acreditar em), even if the English translation uses a different preposition:

• Pensei numa ideia. (I thought of an idea).
• Eu acredito num mundo melhor. (I believe in a better world).

3. Regional Differences & Formality:

• European Portuguese: While classical grammars classify the contraction as optional, in practice, failing to contract 'em' and 'um' is highly unnatural and universally perceived as a mistake by native speakers.
• Brazilian Portuguese: The contraction num is standard and perfectly correct in both formal and informal contexts. However, there is a widespread myth in Brazil that num is informal, leading many professionals to incorrectly 'correct' it to em um in formal texts.

4. Quantity vs. Article:
The contraction num can be used whether 'um' acts as an article ('a/an') or a numeral ('one'). For example: Todo o trabalho foi feito num só dia. (All the work was done in just one day). However, when 'um' is strictly a numeral used in contrast, formal writing prefers the uncontracted em um (e.g., em um ou dois dias). Furthermore, the contraction is avoided in fixed expressions of ratio or proportion. For example, dois em um (two in one) is generally not contracted to dois num.

5. Indefinite Pronoun:
The word 'um' can also function as an indefinite pronoun (meaning 'one' standing alone). The contraction num still applies in this case. For example: Havia vários carros, e eu entrei num. (There were several cars, and I got in one). Note that if 'um' is preceded by words like 'cada' (each) or 'qualquer' (any), the contraction is broken up (e.g., em cada um, not num cada um).

6. Distinction:
Do not confuse num (in a) with no (in the). Use num for general things and no for specific things.

7. Titles Exception:
Prepositions are not contracted with articles that are part of a proper title. For example: Li isso em Um Conto de Duas Cidades. (I read that in A Tale of Two Cities). However, in spoken, everyday Portuguese, native speakers frequently ignore this rule and contract prepositions with titles anyway (e.g., Li isso num Conto de Duas Cidades).

8. Colloquial Brazilian Portuguese:
In spoken and informal written Brazilian Portuguese, num is an extremely common phonetic reduction of não (e.g., Eu num sei for Eu não sei). Do not confuse this slang for 'not' with the contraction for 'in a'.

9. The Infinitive Subject Rule:
There is a strict grammatical exception in both Brazilian and European Portuguese: 'em' and 'um' cannot be contracted if 'um' is the subject of a following infinitive verb. For example: O problema consiste em um de nós sair. (The problem consists in one of us leaving). Because 'um' is the subject doing the action of leaving, it cannot be fused with the preposition.