Adversative conjunctions join two elements when the second idea opposes the first, corrects it, or limits it. In a sentence like Quería salir, pero llovía, the first part presents an expectation and the second one restrains it. If the conjunction is removed, the two ideas are left separate and the contrast relationship is lost. Within the Conjunciones, the adversatives are used to show a clash between two contents, not to add as the Conjunciones Coordinantes nor to explain cause as the Conjunciones Causales.

Basic sense of adversative conjunctions

Idea
Adversative conjunctions join two ideas with contrast.
They can also mark opposition between two facts.
Sometimes they serve to rectify a previous idea.

Pero introduces a direct opposition. It joins words, phrases, or clauses: es pequeño pero fuerte, quiere café pero no té, Llamé, pero no contestó. When it joins two clauses, it is usually set off by commas if the pause is natural: Quise ayudarte, pero llegué tarde. It can contrast tastes, facts, or expectations. It does not correct a negation, it only presents a contrast between two already affirmed ideas or an assertion and its limit.

Common uses of but in contrast

Usage	Explanation
Contrast between words	It is used when it opposes two words or short expressions.
Contrast between phrases	It is used when it opposes two groups of words.
Contrast between sentences	It is used when it joins two complete sentences with opposition.

Sino appears after a negation and corrects the preceding part. The structure is no + verb + sino + corrected element: No quiero té, sino café. The second element does not carry a conjugated verb when correcting a noun, an adjective, or a phrase. When the correction includes a conjugated verb, sino que is used: No llegó tarde, sino que se fue antes. It is also used sino que when the previous clause contains a verb and the second presents another complete action. In Conjunciones Subordinantes you will see other ways of linking sentences, but here the idea is to rectify what is negated.

Correction after a previous negation

Idea
We use sino to correct a negated statement.
We use sino que when a conjugated verb follows.
We do not use sino if no previous negation appears.

Aunque unites two ideas when one does not prevent the other. It can express contrast or concession: Aunque llueve, salimos; Vendrá, aunque esté cansado. The sentence with aunque can come before or after the main one, and the order changes the emphasis, not the basic sense. If the idea with aunque is presented as real, it usually goes with the indicative verb: Aunque hace frío, vamos. If the situation is taken as hypothetical or not confirmed, the subjunctive appears: Aunque haga frío, iremos. Its value is different from that of the Conjunciones Consecutivas, because it does not express a result, but a concession.

Nuances of aunque in contrast

Usage	Explanation
Real concession	It is used when a difficulty does not prevent the action.
Soft contrast	It is used when it presents a contrary idea without denying the other.

Sin embargo and no obstante introduce a more formal opposition and usually appear between pauses or at the start of the second sentence: Estaba cansado. Sin embargo, siguió trabajando; No obstante, aceptó la propuesta. They work well in written texts, news articles, and careful explanations. They do not join two words like pero; they connect sentences or broad segments. When they appear at the beginning, they are separated by a comma. They are often used to mark that the second idea remains despite the first, something very useful in the links that are also studied in Conjunciones correlativas and Conjunciones Comparativas, where the contrast also organizes meaning.

Formal value of oppositions

Usage	Explanation
Formal opposition	It is used when one wants to introduce a more elegant contrast.
Connection between statements	It is used when it joins two ideas with a more written or careful tone.

Use but to oppose two ideas without correcting the first: Quise ir, pero no pude. Use sino to substitute a negated element by another: No vino Ana, sino Marta. Use sino que when the correction introduces a conjugated verb: No estudió, sino que trabajó. Use although when one idea contrasts with the other without preventing it: Aunque estaba ocupado, llamó. Use however and nevertheless when you want a more formal contrast, especially between complete sentences: Era difícil; sin embargo, lo lograron. The choice depends on whether you contradict, rectify, or concede.

Selection according to the nuance of contrast

Idea
We choose pero when we only want to oppose two ideas.
We choose sino when the second idea corrects the first negated.
We choose sino que when the correction includes a conjugated verb.
We choose aunque when the second idea does not negate the first.
We choose sin embargo or no obstante when we want a more formal contrast.