How much power do the devil and his demons have over our thoughts? Can they actually place thoughts into our minds?
Demons can influence our thoughts much in the same way as other human beings can. The only difference is that other human beings can only observe our behavior and hear our words, but demons can see a bit deeper; namely, they can see our imagination and memory and the feelings that come from them. So they are able to tempt us more precisely, you might say, than another human being, since they can perceive the images we carry within.
Even so, demons cannot reach to our strictly spiritual faculties—that is, our reason and our will. These can be moved directly only by God and by ourselves. So when a demon tempts us, he is trying to assure himself that we will fall away from God, but the demon can never be sure that we have lost God's grace or were fully guilty for the sin we fell into under temptation. We must remember that the demons are more desperate than they are powerful.
It is also essential to keep in mind that we have guardian angels who constantly work to move our imagination and memory and feelings toward God and away from sin. Since these good angels see us in the light of the vision of God that they possess, their work in us is much more effective and accurate than that of the demons, most especially if we invoke our guardian angels and open ourselves to their influence by seeking to do the will of God and avoid the occasions of sin.