Complex sentence: Ted calls Lily a "birthday brat" not understanding why she is so upset over ruining "a couple of pictures" so Lily reminds him of their time in Paris, a time reserved for only her and Marshall, but Ted came along with Karen.
Split into simpler sentences: Ted calls Lily a "birthday brat", not understanding why she is so upset over ruining "a couple of pictures". It was initially reserved for only her and Marshall, but Ted came along with Karen.