Here is an example showing that even simple elementary looking problems
can lead to difficult proofs. Harald Helfgott has proven in 2014 that
Every odd integer larger than 5 is the sum of three primes.
The proof is available on the arxiv, had been accepted in a journal but Helfgott
decided to write a book about it instead giving more details. Indeed, this is a mile stone
in Mathematics. It seems as if experts in the field believe however that there is still
a large way from the weak Goldbach problem to the actual Goldbach problem stating that every
even number larger than 2 is a sum of two primes.