it is easy to learn japanese 填空first,but it becomes more and more difficult later on.

it is easy to learn japanese 填空first,but it becomes more and more difficult later on.
it is easy to learn japanese 填空first,but it becomes more and more difficult later on.

it is easy to learn japanese 填空first,but it becomes more and more difficult later on.
起先at first

at first 最初的意思。这里用作时间状语。和later on （后来）刚好相对。