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I've found this question across my JLPT N4 practice

> 1) 楽しかったですよ。参加（できなくて/できないので）、残念でしたね.

The answer is できなくて What are the differences between ので and て in usage for
reasonings?
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2017-09-23T04:22:18.990
53440
2017-09-25T03:05:47.397
25732
question
grammar て-form
Difference between ので and て Form?
The の simply adds an explanatory tone to the sentence.

> 今忙しいので、どうぞお先に行ってください。 (As I am) busy right now, please go on ahead.

But I think you're veering away from the original grammatical matter being
tested in your practice sentence. macraf is giving you a good hint.
CC BY-SA 3.0
2017-09-25T03:05:47.397
53463
2017-09-25T03:05:47.397
25446
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answer