They've got the gardens open by invitation.

他们有花园，对受邀者开放。

快乐的休息日下午 - SpeakingMax英语口语达人

you could leave your response to the invitation slightly open-ended by saying you're not sure.

你可以表示你不确定，开放性地回复这个邀请。

I don't know 课堂 - SpeakingMax英语口语达人

one very natural and very polite way to send an invitation is by using the "I'd like" pattern.

一种非常自然并且礼貌的邀请做法是使用“I'd like”句型。

I'd like 课堂 - SpeakingMax英语口语达人