Original thread: [1]

Question: What exactly is the difference between しかし and でも? they both mean "but"...

In brief

1. しかし is considered to be more formal.
2. しかし is shortened from しかしながら, while でも　is just particular case of ～ても.

A longer explanation

As a "contradictory conjunction" (逆接の接続詞), でも is more colloquial, and always put in the beginning of a sentence. That is, you will often find a Japanese period "。" ahead of でも, when it functions as a contradictory conjunction.

彼は新しい、いい車を持っている。でもめったに乗らない。
(He has a nice new car, but he seldom drives it.)

On the other hand, しかし is more of a written language (書き言葉), or being spoken in a formal occasion, such as a seminar or a formal speech. And it does not have to be placed at the beginning of a sentence, meaning it can be used in a clause (with a Japanese comma ahead of it) i.e.

手術中に病院が停電、しかし 携帯電話の明かりで無事成功。
(During the surgery, the blackout hit the hospital, however, by the lights of cell phones, the surgery was completed safely.)

しかし can be used in casual dialogues too; i.e. when you try to change the subject:

A: ずいぶん、ひごい雨ね。(Whoa, that's some heavy rain.)
B: しかし それにしても 佐藤さん、遅いね。(Well, don't you think Satou is still slow despite this?)

The difference with ただし

ただし is more about partial negation. It adds some conditions/restriction to what had been said in the sentence before it.

In other words, ただし　would introduce a proviso, akin to "subject to, provided that". For example, a situation in English, where there is a sign at store:

"All credit cards welcome, however an extra 2% processing charge applies to Amex" etc.

There is no an exact equivalent of it in English other than for "however", which is why it gets confused with でも, しかし, etc.