ただし
however, but (a condition or exception)
Statement。ただし、condition / exception。
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ただし adds a condition or an exception to what was just said: 入場は無料です。ただし、予約が必要です, "entry is free. However, you need to book".
It's the "terms and conditions" but. The first sentence is generally true or allowed; ただし narrows it: 写真を撮ってもいいです。ただし、フラッシュは使わないでください.
It's common on notices, in rules, instructions and explanations. It doesn't contradict the first sentence, it limits it.
Compare しかし, a general "but" that can contradict, and ところが, which describes an unexpected turn of events. In a reading passage, ただし usually introduces the detail that matters most, so it's worth slowing down for.
Examples
明日は休みです。ただし、午前中は電話に出ます。
I'm off tomorrow. However, I'll answer the phone in the morning.
入場は無料です。ただし、予約が必要です。
Entry is free. However, you need to book.
写真を撮ってもいいです。ただし、フラッシュは使わないでください。
You may take photos. However, please don't use the flash.
毎日営業しています。ただし、祝日は休みです。
We're open every day. However, we're closed on public holidays.
辞書を使っても構いません。ただし、電子辞書はだめです。
You may use a dictionary. Electronic dictionaries aren't allowed, though.