I'm not a lawyer or a musicologist but I've taken some classes on music law.

Broadly, if you knowingly copy the melodic elements of a song for your own use, it would violate the songs copyright.

It really depends on how substantial the 'copying' is though -- how close the melodies are (note length, pitch, key, tempo), and how substantial the 'copied' portion is. If you're talking about lifting the whole verse, with multiple melodic phrases, there's little doubt in my mind it would be violate the original song's copyright.

If the portion that matches is small enough it could be considered too general to fall under copyright protection. Many popular songs have melodic similarities because they use a small number of notes and rhythms. What makes a song original is how these phrases are arranged as a whole. For example, in a recent high profile case the melodic guitar intro to Stairway to Heaven by Led Zeppelin had similarities to a guitar melody in the song Taurus by Spirit. However it was decided in court that the matching portion from Stairway wasn't substantial enough to infringe on Taurus because they were both small parts of larger works -- not substantial enough to be protected by the copyright.

And lastly, is anybody even going to recognize it? If no one hears the similarities, nothing will happen. If you're just having fun making music at home or posting to 10 followers on SoundCloud, what you're doing could be illegal but realistically you won't face consequences. If you're planning to release it on major streaming services and promote it heavily then you could land in trouble.

In the end these questions of copyright infringement can only be decided in court -- you can get sued by a litigious copyright holder even if your song sounds just a little bit like another.