Complex sentence: Many Poles in Lithuania in passports were signed as Lithuanians, therefore they also were forced to attend Lithuanian schools, since then the number of schools gradually decreased to 9 in 1940.
Split into simpler sentences: Many Poles in Lithuania were signed in as Lithuanians in their passports, and as a result they also were forced to attend Lithuanian schools. The number of Polish schools gradually decreased to 9 in 1940.