A lot of flowers have been planted in the garden.用一般过去时或一般现在

虽然时态可以变,但是,意思不一样的.
按照你提供的句子,A lot of flowers have been planted in the garden.
谓语用的是现在完成时,意思是：花园里种了许多花.（强调的是动作）
如果改为一般过去式,即 A lot of flowers were planted in the garden.
翻译为中文是：花园里种过许多花.（强调的是过去的状态,言下之意
是,现在没有种花.）
如果改为一般现在时,即：A lot of flowers are planted in the garden.
译为中文：花园里种有许多花.（强调的是现在的状态）

可以：
A lot of flowers were planted in the garden.
A lot of flowers are planted in the garden.