El candidato responde a cada pregunta con calma.

With the meaning “to answer (a question),” Spanish commonly uses responder a + thing: responder a una pregunta. You can also use responder without an object if context makes it clear: No responde. In much of Latin America, the direct-object form responder la pregunta is also widely used and accepted. In careful or formal Spanish, responder a la pregunta is often preferred. So all of these can be heard:

• Responde a la pregunta.
• Responde la pregunta.
• Responde.

Yes. Contestar is a near-synonym of responder in this sense. Both are standard. With contestar, both patterns are fully standard:

• Contesta a cada pregunta.
• Contesta cada pregunta. Use whichever you prefer; in some regions, contestar sounds a bit more colloquial, but it’s perfectly fine.

It’s the present indicative, third person singular of responder. It can describe:

• An action happening right now: “is answering” (often Spanish uses the simple present for this).
• A habitual/general action: “answers (whenever asked).”

Change the noun and article to feminine:

• La candidata responde a cada pregunta con calma.

Yes. Spanish word order is flexible with adverbial phrases. All of these are natural:

• El candidato responde con calma a cada pregunta.
• Con calma, el candidato responde a cada pregunta. (more stylistic)

It’s very idiomatic and common. Alternatives:

• tranquilamente
• con tranquilidad
• less common but correct: calmadamente They all work; con calma and tranquilamente are especially natural.

You can, but it changes the feel slightly:

• Responde con calma / tranquilamente describes the manner of answering (adverbial).
• Responde calmado / tranquilo describes the candidate’s state while answering. It’s acceptable, but the adverbial options are more neutral for “answers calmly.”

With the responder a + thing pattern, replace the noun phrase with a demonstrative or a pronoun phrase:

• El candidato responde a cada una con calma.
• If referring back to previously mentioned questions: El candidato responde a ellas con calma. Note: We normally don’t use le/les for inanimate “a + thing” here. Use a eso / a ella(s) instead.

Treat it like a direct object:

• El candidato la responde con calma. (referring to la pregunta)
• Plural: El candidato las responde todas con calma. With compound verbs, clitic placement follows the usual rules:
• La va a responder / Va a responderla

Make both subject and verb plural, and keep agreement throughout:

• Los candidatos responden a cada pregunta con calma. If the addressee is plural questions, you still use singular after cada: cada pregunta.