Plz help for Spanish class

Which sentence uses the verbs correctly?

What is a verb infinitive and what does is mean to conjugate a verb?

On what political idea did president truman base the marshall plan? as economic aid increased in europe, the friendship of nations with the united states would increase. as peace throughout the world increased, the threat of another world war would decrease. as prosperity in europe increased, the threat of communism spreading would decrease. as the power of capitalism increased in europe, stalin's power in the soviet union would decrease.