Jasper has put up no parking signs outside his gate, but these have not had any effect.

Question

为什么前后两句都用完成时.

Answer 1

“Jasper已经贴了禁止停车的标示”，用现在完成时，表示已经贴了，动作已经完成。

“但还是没用任何成效”，已经贴了，但是没成效，强调对现在造成的结果和影响。所以还是用了现在完成时。

Answer 2

句子的时态主要要看句子的意思来定。
前后文意思连起来看看。

Answer 3

已经put up no parking signs，
have not had （没有，“已经”证实的）any effect.

因此得用完成式。