- <em>Tenemos que seguir estas reglas </em><u><em>para</em></u><em> salvar el medio ambiente.</em>
Explanation:
The translation of the sentence is:
- We have to follow these rules <u>to</u> save the environment.
The word <u><em>"para" </em></u>has many uses in Spanish within which we can mention:
A time limit
The movement towards a place or event.
Recipient of an action.
Opinion regarding a specific topic.
<u>Purpose of an action</u>.
It is precisely the latter mentioned that is applied in the sentence, since the purpose of following the rules (as mentioned in the first part of the compound sentence) is to save the environment, so it is appropriate to use the word <u><em>"para."</em></u>