agreement

1. It should be 'a sand castle' instead of 'the sand castle'

2. It sounds weird to me. Why dont you say 'on the beach' instead of 'in the pool'? We normally do not have sand in swimming pools.

3. We'll go swimming in the pool.
+ We'll build a sand castle
= We'll go swimming in the pool and (We'll) build a sand castle.

When we use a conjunction (and) to link two sentences together, we delete the duplicated words (We'll).

第一句會係正確
因為will後面個verb係唔駛轉tense既,而go swimming, go係verb 而唔係swimming,所以你個build係唔駛轉tense