1. We did cardiopulmonary resuscitation to the wouded.

2. We gave first aid to the injured.

(Are they both grammatical? Do we have to use 'to'? What does 'the wounded' and 'the injured' mean? Singular or plural? Thank you.)

Both are grammatically correct.

The wounded and the injured both mean people who have been hurt. As nouns, they are both plural.

Both sentences are grammatically correct. The use of 'to' is optional in these cases.

1. "We did cardiopulmonary resuscitation to the wounded."
This sentence means that the individuals performed cardiopulmonary resuscitation on someone who was wounded. 'The wounded' refers to people who have suffered injuries or wounds.

2. "We gave first aid to the injured."
This sentence means that the individuals provided first aid to someone who was injured. 'The injured' refers to people who have suffered physical harm or injury.

In both cases, 'the wounded' and 'the injured' can refer to either a singular person or multiple people. It depends on the context of the situation being described. For example, if multiple people were injured, you would say "the injured," but if only one person was injured, you would still use "the injured."