The infinitive is used in Spanish in specific situations, generally when a command is directed to the general public through signs, for example in traffic, in towns, in public areas. It is also used when we are giving step-by-step instructions, for example in a recipe book, or when explaining how to build something.
If it is a negative command, you just place "No" in front of the infinitive. So there is no need to conjugate the verb.
The Spanish infinitive in signs
Here are some examples:
Apagar las luces.Switch off the lights.
No pisar el césped.Keep off the grass.
Precalentar el horno.Preheat the oven.
Cocinar a fuego lento.Cook on a low heat.
The Spanish infinitive for step-by-step instructions
Doblar la hoja por la mitad.Fold the paper in half.
Regar la planta con agua abundante.Water the plant with plenty of water.
The use of the infinitive as a command instead of the imperative makes the command a "softer" and more "polite". You will still sometimes find commands directed to the general public in the imperative, but the infinitive seems to be preferred in these cases.
See also Using preposition a + infinitive to express a command in Spanish
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