Learn Hardcore Spanish (Latin America): Es necesario que la ventilación funcione bien durante enero. - It is necessary that the ventilation work well during January.

Because the expression es necesario que expresses necessity, which triggers the use of the subjunctive mood in Spanish for the subordinate verb. When you talk about requirements, wishes, or doubts, Spanish uses the subjunctive in the dependent clause.

What type of expression is es necesario que?

It is an impersonal expression of necessity. Impersonal expressions start with es + adjective and always require que plus a subordinate clause in the subjunctive.

Can we drop que after es necesario?

No. The conjunction que is mandatory to link the impersonal expression to the subordinate clause. Without que, the sentence is ungrammatical.

What does durante mean here? Could we use en instead?

durante means “during,” emphasizing the entire period.
en enero is also correct but more neutral; it usually refers to a point or general time within January rather than stressing duration.

Could we use por enero instead of durante enero?