They apparently became together in the second half of 2012. When Swift was spotted in the fall wearing a paper aeroplane necklace (which had long since become a signature of Styles'), rumours about their potential romance began to circulate. In December 2012, they were captured on camera strolling together in the Central Park Zoo.
"I knew you were trouble (IKYWT)", "Out" and "Style" of the Woods are the three songs about Harry Styles that are the most obvious. Since she openly praised him for being present and serving as the song's inspiration when she accepted the VMA for best female video for IKYWT, we can conclude that the song is about Harry.
It is important to note that Swift has never confirmed that any of these songs are about Styles. However, the lyrics of these songs do seem to suggest that they were inspired by her relationship with him.
In addition to these songs, there are also a few other songs that are rumored to be about Styles. These include "Clean," "How You Get the Girl," and "I Know Places."
Ultimately, it is up to the listener to decide whether or not these songs are about Harry Styles. However, there is no doubt that Swift's relationship with Styles had a profound impact on her music.
Taylor Swift image Instagram (@elitecoachtransportation)