William Butler Yeats is widely considered one of the greatest poets in the English language and he had a significant impact on modern poetry. Here are some key ways in which Yeats contributed to modern poetry:

1. Innovative Use of Symbolism and Allusion: Yeats was a master of symbolism and allusion, with many of his poems containing complex systems of symbols drawn from a wide variety of sources, including Celtic mythology, Christian mysticism, and the occult. This innovative use of symbolism and allusion became a defining characteristic of much modernist poetry.

2. Exploration of the Subconscious: Like many modernist writers, Yeats was interested in exploring the inner workings of the human mind, particularly the subconscious. His poetry often delves into dreams, visions, and other aspects of the subconscious, helping to pave the way for the stream-of-consciousness techniques used by later modernist writers.

3. Focus on the Individual: Yeats' poetry often focuses on the individual's experience in a rapidly changing world. This emphasis on the individual perspective and personal experience is another hallmark of modernist literature.

4. Experimentation with Form: Yeats was not afraid to experiment with poetic form. He often combined traditional forms with innovative techniques, creating a unique poetic style that was both rooted in tradition and forward-looking. This willingness to experiment with form had a significant influence on later modernist poets.

5. Political Engagement: Yeats' poetry is deeply engaged with the political issues of his time, including the struggle for Irish independence. His ability to weave together the personal, the political, and the mythological in his poetry had a profound influence on the development of modernist poetry.

In summary, Yeats' innovative use of symbolism and allusion, his exploration of the subconscious, his focus on the individual, his experimentation with form, and his political engagement all had a significant impact on modern poetry.