I have two questions on complex geometry.

First one is that why the existence of almost complex structure on tangent bundle on real 2n-dimensional manifold is a topological question?

Wikipedia describes it as a topological question. I think that mean there is some homology or cohomology group associated to topological property whose vanishing or nonvanishing pertains to the existence.

Would you explain why it should be a topological question intuitively and may I suggest exact topological invariant which captures the existence property?

Second, in Huybrechts's book 'Complex Geometry', when it comes to Euler exact sequence on $P^n$, he mentioned that there is natural inclusion map such that $O(-1)\rightarrow\oplus_{j=0}^{n}O$. I can hardly come up with any idea on what this inclusion map is.

If possible, would you let me know the exact expression for this map? (here, $O(-1)$ is the tautological line bundle sheaf on $P^n$ and $O$ is the holomorphic sheaf of the trivial line bundle)

Hoping to get some shedding light in your reply.