Coordinating conjunctions join elements of the same level: two words, two groups of words or two clauses that perform the same function. In a list, they connect nouns, adjectives or verbs: pan y queso, alto y fuerte, trabaja y estudia. They can also join complete sentences: Llegó temprano y abrió la tienda. Each part retains its independence; the conjunction only relates them. If the conjunction is missing, the parts are left separated and the phrase loses that relation of sum, contrast, alternative or correction. To see how the meaning changes according to the link, it is useful to compare them with other classes of Conjunciones and with the subordinating ones, which do not coordinate elements of the same level.

Basic function of coordinating conjunctions

Usage	Explanation
Union of words	It is used to join two words of the same level within a simple sentence.
Union of groups of words	It is used to join groups that function as a single piece in the sentence.
Union of sentences	It is used to connect two complete sentences that have the same importance.

Y adds an element to another. Connects people, objects, actions or ideas without changing the logical order of the sentence: comí y dormí, libros y cuadernos, madre y padre. Before words that begin with i- or hi- the word e is used to avoid clashes in sounds: padre e hijo, salud e higiene, agua e hielo. The form changes only by the initial sound of the following word, not by its meaning. It is kept y in front of other vowels and in front of h when the word does not begin with the sound i: mesa y almohada, arte y historia. In long unions, y can coordinate two clauses and can also appear together with the Conjunciones correlativas when the sum is repeated in larger series.

Use of y and change to e

Idea
Use y to add an element to another when there is no sound clash.
Change y to e before words that begin with i or hi to avoid a difficult sound.

O expresses alternative, choice or possibility. It presents two options and leaves the selection open: té o café, vienes o te quedas, hoy o mañana. Before words that begin with o- or ho- the form u is used to make pronunciation smoother: siete u ocho, madre u hormiga, esto u otro. The form changes only by the initial sound of the following word. u is not used before every vowel, only before o or ho at the start of a word. When the phrase offers more than two alternatives, the conjunction continues to mark each choice: casa, piso u oficina. In more complex sentences, the choice can be linked to Conjunciones subordinantes if an option depends on a condition or a prior decision.

Use of o and change to u

Idea
Use o to express an alternative, choice, or possibility between options.
Change o to u before words that begin with o or ho so it sounds better.

Ni coordinates elements within a negative structure. It joins two words, groups or sentences and reinforces the negation already present: No vino ni llamó, ni frío ni calor, no quiere café ni té. It can also lead a negative sequence when the sentence begins with that idea: Ni tú ni yo sabemos la respuesta. In that case, the whole construction keeps a negative meaning. Ni does not introduce a neutral alternative like o; it eliminates options or adds them as absence. It appears frequently in repetitions of negation to add emphasis: No habla, no escucha ni responde. Its function fits well with broad negative structures and with the Conjunciones adversativas, because both deal with contrast or rejection, although ni does so from negation.

Use of ni in negative structures

Usage	Explanation
Negative addition	Use to add another element within a negation without breaking the negative sense.
Intensification of the negation	Use to insist that something did not happen and reinforce the negative idea.
Coordination of two negations	Use to coordinate two negative parts that are joined by the same sense.

Pero connects two ideas that oppose each other in a gentle way. The first part sets up an expectation and the second limits it, corrects it or contrasts it: Quise ir, pero llovía; es pequeño, pero rápido. It is the most common form in conversation and in general writing. Mas expresses the same contrast, but sounds more formal or literary: quise ayudar, mas no pude. It is not commonly used in everyday speech. Both coordinating conjunctions connect elements at the same level, but pero appears in neutral registers and mas in more careful or expressive registers. For strong contrast, the language usually prefers other resources from Conjunciones adversativas, though pero remains the central form.

Contrast with pero and mas

Usage	Explanation
Common contrast	Use pero to oppose one idea to another in a casual, natural style.
Formal contrast	Use mas to mark contrast with a more literary or refined tone.

Sino appears after a negation to correct what was said before and to substitute one option for another: No quiero té, sino café; no estudia medicina, sino derecho. The first part denies a possibility, and sino introduces the correct or true one. It is not used to sum ideas nor to oppose two independent facts. The usual structure is negation + sino + corrected element. When the second part carries a verb, the full form can also correct an action: No vino ayer, sino hoy. In contrast to pero, which only contrasts, sino replaces. That corrective function relates to the opposition mechanisms of the Conjunciones adversativas and to the Conjunciones Causales when the sentence explains why one option is discarded.

Corrective use of sino

Idea
Use sino after a negation to correct the previous idea.
Use sino to substitute a false option with the correct one.

Some coordinations use two particles spread through the phrase to present alternate or repeated options. Bien... bien offers two possibilities that alternate: bien llueve, bien sale el sol. Ya... ya marks alternation or succession: ya ríe, ya llora; ya en casa, ya en el trabajo. Ora... ora has a similar, more literary or elevated value: ora canta, ora calla. These pairs place one part before each coordinated element and maintain the idea of alternation throughout the construction. They do not join by sum, but by repetition of options. Their function is similar to that of other Conjunciones correlativas, because they distribute the relationship between two or more sentence segments.

Coordination with correlative pairs

Usage	Explanation
Repeated alternation	Use bien... bien to present options that take turns.
Ongoing change	Use ya... ya to show that something changes from one way to another.
Sequential distribution	Use ora... ora to list movements or ideas that succeed one another.

Among long coordinate sentences, the comma helps separate the parts and read them clearly, especially when each sentence has its own subject, verb and complements: Llegó tarde al aeropuerto, pero alcanzó el vuelo. With y, o and ni the comma is usually omitted if the parts are short: Compró pan y leche. If the coordinate clauses are long or already carry internal commas, it is advisable to mark the pause with a comma before the conjunction: Terminó el informe, y después llamó a la oficina. With pero and sino the comma is very common when joining complete sentences. Punctuation does not change the coordinating relation; it just organizes the text better and avoids ambiguities, just as happens in other constructions with Conjunciones consecutivas or Conjunciones Comparativas when the phrase needs to clearly mark each segment.

Comma in long coordinated sentences

Idea
Often place a comma before the conjunction when the coordinated sentences are long and help reading.
Use a comma to separate extended coordinated parts and prevent the sentence from becoming confusing.
Comma improves clarity when both coordinated sentences have a lot of information.