The Word of God shows us the will of God and helps us to pray effectively.

1 John 5:14-15 "And this is the confidence that we have in Him, that, if we ask any thing according to His will, He hears us:
And if we know that He hears us, whatsoever we ask, we know that we have the petitions that we desired of Him."

Salvation, healing, deliverance, prosperity, love, joy, peace, and so much more is God's will for us; so we can pray confidently, knowing that He hears us and that we will receive what we ask. Of course it has to be according to His will.

In other words, we can't as for somebody else's husband or wife or anything that belongs to someone else. We can't ask for someone death or disaster or anything that may harm someone else. That all belongs to satan. 

John 10:10 "The thief [satan]comes not, but for to steal, and to kill, and to destroy: I [Jesus]am come that they might have life, and that they might have it more abundantly." 

John 10:10 distinguishes what is from the devil and what is from God.