have done before
Structure
- Verb[た] + こと・が・ある
Why is this sentence:
あそこの公園（こうえん）に あそびにいったことがない でしょう。
This option is not valid?
あそこの公園（こうえん）に あそびにいったことがありません でしょう
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Hey 
It is not valid because でしょう follows short forms. By people younger than 50-60 years old it will be considered unnatural.
So, あそこの公園に あそびにいったことがない でしょう。
and あそこの公園に **あそびにいったことがありません(no でしょう)。 are correct.
More here: がいる and がいます
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Thanks for the clarification. The でしょう entry doesn’t seems to especify that and I didn’t thought oj compare with the external resources.
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No problem 
I will write a warning about it in the でしょう lesson. 
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Hi, just echoing madmalkav’s point above.
I ran into this issue this evening, but couldn’t find any mention of the warning on the でしょう entry.
Sorry about that! We have added a note in でしょう’s structure that indicates that でしょう follows the short form of a verb. Thank you for your patience! Cheers!
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Hey, I don’t quite understand the nuance between using just the past tense and adding こと・が・ある
To me the meaning seems to be the same, if you only use the past tense, doesn’t it already implies that you’ve done (or not) said action ? What does using こと・が・ある add to what you’re saying ?
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Hey
たことがある expresses that someone has (ever) experienced doing something before. It also implies that the event is not fresh, happened further in past.
The difference is easier to notice if we think about it from a wider perspective:
ことがある and normal た form answer to different questions.
日本に行ったことがある？
Have you ever gone to Japan?
うん、日本に行ったことがある。
Yes, I have experienced going to Japan.
Yes, there was a time when I went to Japan.
日本に行った？
Did you go to Japan?
うん、日本に行った。
Yes, I went to Japan.
Is it a bit more clear now?
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Yes, I think I got a better grasp on the nuance. 
Thanks again mrnoone !
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This is another grammar point where you should probably also mention the 漢字 spelling:
た事がある
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Hey 
Rather than in たことがある or ることがある construction, 事 is used most of the time when refering to some untangible “thing”.
日本語を自分で学ぶことがよくある。
I often study Japanese by myself.
おじちゃんからまだ学事がある。
There are still things I can learn from grandpa.
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For the negative sentences, should ことはない be accepted in addition to ことがない?
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Sorry for being super late with answer! 
The answer is yes, は is very often used with negatives since it expresses contrast and therefore puts more emphasis.
For example：
While
あそこの公園にあそびにいったことがないでしょう。
would be statement:
You have never gone to play at that park over there, have you.
あそこの公園にあそびにいったことはないでしょう。
Would have nuance:
You have never gone to play at that park over there, have you (but you went to other ones).
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Can you add that as an alternate answer to the relevant sentences then?
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I added those before writing the answer
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Hi, I’ve a question to make sure I understand this bit.
In “I have read this book before (この本を読んだことがある)” I thought it could be この本を前に読んだ. But as I now understand 前に is more specifically used when it’s before X in time or space, correct? So in this instance it’d be like saying “I have read this book before …” where you end your sentence with some kind of cliffhanger.
During my reviews, this sentence came up.
‘結婚したことがありますか’
Fair enough.
But my answer of
結婚をしたことがありますか
This was rejected. Am I missing something? I thought you could use an を after a suru noun.
Have I misunderstood something?
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@Matt_RWR
Hey Matt,
I have added it as an alternative answer.
The meaning is the same.
The difference is that when you use するVerb + する you cannot modify (describe) the するverb with noun or adjective, with するVerbを + する you can.
Example:
数学の勉強する not natural
数学の勉強をする natural
幸せな結婚する not natural
幸せな結婚をする natural
But you can say:
数学を勉強をする natural
Cheers
Ah - I never knew that. Thanks for the info and for adding the alternative!