‘Se não’ These are two distinct terms (“if” is a conditional conjunction, pronoun or passivating particle + “not” which is a negative adverb) which together always express a condition. This can be seen in the following examples:
(PT: “Se não te portares bem, ficas de castigo!” / EN: “If you don’t behave, you’re grounded!”)
(PT: “Se não comeres, não há futebol para ninguém.” / EN: “If you don’t eat, there’s no soccer for anyone.”)
‘Senão’ It is an adverb meaning except, not unless, unless, less. It is a conjunction that means when not, otherwise, contrary. It is a masculine noun that can mean defect; problem; flaw; blemish. ‘Senão’ it is a single word, and can be a noun, conjunction, adverb or preposition.
The term ‘Senão’ is synonymous with: otherwise (caso contrário), in other words… (de outro modo…)
(PT: “Apressa-te, senão perdemos o autocarro,”; / EN: “Hurry up, otherwise (senão) we’ll miss the bus”).
The term ‘Senão’ is synonymous with: but (mas), however… (porém…)
(PT: “Não na cidade, senão no campo.”; / EN: “Not in the city, but (senão) in the country”).
The term ‘Senão’ is synonymous with: except (exceto), unless (a não ser), otherwise…
(PT: “Ninguém disse nada senão o pai.”; / EN: “No one said anything but (senão) Dad”).
The term ‘Senão’ is synonymous with: defect…
(PT: “Não há bela sem senão.”; / EN: “There is no beauty without a catch (senão) “).
A very practical and simple way to figure out when to use each of these forms (senão or se não) is to replace the word in question with another expression such as “if not” (caso não).
In case the substitution is not possible, does not make any sense, then this reality allows you to conclude that it is better to choose, to use the word “else” (senão).
Whenever this substitution for “if not” (caso não) is possible and works, it means that the form you should use is “if not”(se não), separately.