Adversative conjunctions link two ideas to show contrast, opposition or correction. Within the system of Conjunctions, they can coordinate clauses or introduce a relationship of concession and rectification. Some belong to the common register, such as pero and aunque, while others are more formal or literary, such as sin embargo, no obstante and mas.

But coordinates two opposing ideas and is usually preceded by a comma when linking sentences or broad segments. In colloquial speech the comma may be omitted at times, although normative writing it is frequently kept. Its usual position is between two clauses that contrast.

IdeaIdea	ExampleExample
Une dos ideas opuestasLinks two opposing ideas	Quería ir, pero llovió.I wanted to go, but it rained.
Suele llevar coma antes de la conjunciónUsually carries a comma before the conjunction	Estudió mucho, pero no aprobó.He studied a lot, but he didn't pass.
Puede omitirse la coma en registro coloquialMay be omitted in colloquial register	Quise llamarte pero se me hizo tarde.I wanted to call you, but it got late.

Sino corrects or substitutes what is stated before a negation and does not express simple opposition like pero. It appears after an immediate negation and presents the correct element against the negated. When a conjugated verb follows, sino que is used; this alternation is related to the behavior of the Coordinating Conjunctions.

IdeaIdea	ExampleExample
Corrige lo negadoCorrects what is negated	No vino, sino su hermano.No vino, sino su hermano.
Sustituye un elemento por otroSubstitutes an element for another	No quería café, sino té.No quería café, sino té.
Usa sino que si sigue un verbo conjugadoUses sino que when a conjugated verb follows	No vino, sino que llamó.No vino, sino que llamó.

Although introduces a concessive idea and subordinates the clause that begins it, so it relates to the Subordinating Conjunctions. It can appear with the indicative when the fact is presented as real and with the subjunctive when expressing concession, doubt or lack of confirmation. Its usual position is at the start of the concessive subordinate clause.

IdeaIdea	ExampleExample
Introduce una concesión realIntroduces a real concession	Aunque llueve, salgo.Although it rains, I go out.
Puede llevar indicativoCan take indicative	Aunque está cansado, sigue trabajando.Although he is tired, he continues working.
Puede llevar subjuntivo para hipótesis o dudaCan take subjunctive for hypothesis or doubt	Aunque viniera, no cambiaría nada.Even if he came, nothing would change.

However and nevertheless are adversative locutions with a more formal tone than pero, and they usually appear between two sentences or after a strong pause. Mas is equivalent to pero, but belongs to the cultivated or literary register and is infrequent in spoken language. These forms express contrast, though their use and tone vary according to the context.

IdeaIdea	ExampleExample
Sin embargo expresa contraste con tono formalHowever expresses contrast with formal tone	Quiso venir; sin embargo, no pudo.He wanted to come; however, he could not.
No obstante presenta oposición en estilo cuidadoNevertheless presents opposition in careful style	Llovía; no obstante, salió.It was raining; nevertheless, he went out.
Mas equivale a pero en registro literarioMas is equivalent to pero in a literary register	Quiso, mas no pudo.He wanted to come, mas he could not.

The comma is usually placed before pero and mas when linking two independent clauses. Sino does not require a comma after the immediate negation that introduces it, because it is part of correcting what is negated. Although it usually heads a concessive subordinate clause, whereas sin embargo and no obstante can be separated with a semicolon or with a strong pause when linking complete sentences.

IdeaIdea	ExampleExample
La coma suele ir antes de peroThe comma usually goes before pero	Quería salir, pero hacía frío.I wanted to go out, but it was cold.
Sino no exige coma tras la negación inmediataSino does not require a comma after the immediate negation	No quería pan sino fruta.No quería pan, sino fruta.
Sin embargo y no obstante admiten pausa fuerteSin embargo and no obstante admit a strong pause	Estudió mucho; sin embargo, falló.He studied a lot; however, he failed.

Adversative conjunctions serve to oppose, nuance or rectify one idea against another. But they mark direct contrast, sino corrects after negation, sino que accompanies a conjugated verb, although they introduce concession and may require indicative or subjunctive, and sin embargo, no obstante y mas broaden those nuances with different degrees of formality. Their value depends as much on meaning as on position and punctuation.