She looked after them very carefully,and then the summer came,they looked very nice用then是否正确

改成slowly 会好点

可以用then。as也可以，但是要在they looked very nice这里，动词look的形态就要进行相应的变化了，而且应该选用延续性动词。比如，getting better

and then 和then 意思一样，转折连词，然后

用as更好，强调时间慢慢的到来