Difference between 「 ただ」, 「たった」, 「ただ　～だけ」, and 「ただし」


I translate these words as either "only, just, simply".

I'd like to know how I could differentiate these 4 ways (i.e., when I use that
one, and when I use the other).

Examples:

>   * たったひとつの恋 (Dorama)
>   * ただの子供だ
>   * ただ待つだけだ
>   * よし, この 10 万円は君にあげよう. ただし, これ以上はお断わりだよ.  
>  ...  
>  ... so on
>




As far as I know, たった and ただ have exactly the same meaning, and are just
variants, something like ばかり and ばっかり。 As to the third example sentence, I'd
venture to say that the ただ there is the same ただ at the head of the snake. (笑)

By the way, as Mr. Ito points out in comments above, if you meant to say,
"It's just an ordinary snake" or the like, it needs to be ただの蛇だ。 If you meant
"It's only a snake", as in a case where you were expecting a Yeti, it would be
蛇だけだ。

