Gustan should be “gustaron”, no? are you asking about what tense it’s in? ie. past, present, future?
Answer:
"ser" and "estar", these two verbs differ on the condition of permanency of that what is being explained in the sentence.
It is better to use "ser" when you are expressing something that will be permanent, or most likely will be permanent.
it is better to use "estar" to express something that is likely to end in a short or medium term.
sometimes the verbs are interchangeable, in such cases is not needed to explain the length of the state expressed by the sentence.
The third sentence in the questions uses the literal translation from the English present continuous time to the Spanish "présente continuo"
Answer: sería imposible la vida en este planeta ya que los gases del aire son vitales para la vida en la Tierra. los seres humanos y los animales no pueden sobrevivir sin oxígeno y las plantas verdes no pueden sobrevivir sin dióxido de carbono.
Hope this helps you !
