I realized then that they hadn't been heading for a flight,but had just been riding the shuttle.句子成分划分,

I realized then that they hadn't been heading for a flight,but had just been riding the shuttle.句子成分划分,
I realized then that they hadn't been heading for a flight,but had just been riding the shuttle.
句子成分划分,

I realized then that they hadn't been heading for a flight,but had just been riding the shuttle.句子成分划分,
I 是主语；realized是谓语；that从句是宾语.
宾语是由两个并列分句构成的宾语从句.