I just learned some stuff about the use of **だけ** and now I wonder, if I want
to say

`"Just me and a friend did this"` , could I use **だけ** to express this?

If so, how would I have to use it? And when would I have to refrain to using
`一人で`?

This also raises the question whether 一人で could modify a verbal expression
related to a subject which represents multiple persons?

> 1a) これは私と友達がするだけだった。 1b) 私と友達だけはこれをした。
>
> 2a) これは私が一人でした。 2b) これは私と友達が一人でした。

I assume that 1b) is pretty ungrammatical, but I wanted to ask about it
nevertheless because it is stated in my textbook that だけ can be attached to
nouns.

This is in different context though, like here: `休みは日曜日だけです。`

I might be digging too deep now, but I wonder whether だけ and 一人で are one of
these things which don't modify a specific sentence element, but rather the
whole sentence.

Unfortunately I don't know what these are called in English terminology; in my
German linguistics lessons they were called "partikel".

I could give an example here:

> " **Of course** I can do this."
>
> " **Eventually** the machine crashed."
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grammar
The use and placement of だけ
The best way I have been able to translate 「だけ」 has been to refer to limits.

「一人」 means "one person", and you'd probably be best leaving at that.

I think I have heard some people using 「一人で」 in plural, in the sense that the
English "alone" can be used in plural, (clarification) thus

> We did this alone.

can mean "We did this by ourselves." (It can also mean "We did this one
thing." That's not really current usage, however.)

(end clarification) But I'm not sure I heard it right, and I'm not sure that
the people I heard weren't using it in a non-standard sense.

(afterthought)

When there are just two, all alone, you can use the expression 「二人」 or 「二人きり」。

(end afterthought)

(clarification)

I checked with both of my kids (college senior and high school senior) and
they agree that 「一人で」 is not used in the plural sense. See my later comments
on the "individually" meaning.

(end clarification)

(further afterthought)

"Of course" and "eventually" can be interpreted to apply to a whole English
sentence, but you really shouldn't do that. Apply to the verb when in doubt,
and move it there when you need to be exact. Using your examples,

> I can, of course, do this.
>
> The machine eventually crashed.

Now, 「だけ」 does have some uses that may sound odd, but don't go looking too far
away. In other words, if you think it's modifying the whole sentence, you're
misreading it.

(end further afterthought)

Trying to interpret your four sentences,

> 1a) これは私と友達がするだけだった。

This almost sounds to me like you might have been trying to say

> これは私と友達がやるしかないものだった。

or, "This was something that my friends and I just had to do." You probably
did not mean that. Also, consider, 「やるしかなかった」。

Instead, you might have said

> これは私と友達だけがするものだった。

(something we had to do ourselves), or

> これは私と友達だけ **で** するものだった。

(something we had to do **by** ourselves)

Your second sentence,

> 1b) 私と友達だけはこれをした。

comes out as

> Speaking only of my friends and me, we did this.

I think you wanted to use 「で」 instead of 「は」。

> 私と友達だけでこれをした。

But I'm also thinking I have heard

> 私が友達とだけでこれをやった。

Let's see what our Japanese friends who hang out around here have to say about
that.

Continuing,

> 2a) これは私が一人でした。

Ouch. Did you mean past of 「です」 or past of 「する」？

「やる」 is not a perfect replacement for 「する」、 but it comes in handy in places
like this. (And it's a bit preferred over plain 「する」 in some dialects.)

Unless I misunderstand my Japanese, 「でした」 in (2a) will be read as the past
tense of 「です」、 so it will be read,

> In this, I was alone.

However, given what that means, it can and is used as a substitute for

> これは私が一人でやった。

so it doesn't seem like a big deal, but misunderstanding what is happening
there can trip you up.

And, finally,

> 2b) これは私と友達が一人でした。

You really want to use 「しました」 or 「やった」 when saying it this way.

(further afterthought)

But you really don't want to use 「一人」 here.

(end further afterthought)

(afterthought)

And you want to consider the "individual" meaning, as well. 「私たち一人一人」 is "we,
individually". So, even if the plural sense has become accepted as it has in
the English word "alone",

> 私たちは一人でやった。

would more likely be read "each, individually" than "only us".

(end afterthought)

But try this on for size:

> これについて私と友達は一致していました。

meaning

> My friends and I were united in this.

And this one:

> これは私と友達 **一人** でやったのです。

meaning

> This was something I and **a** friend did.

One friend.

Okay, if you and some friends and some others did "this", you can use 「や」 and
say,

> 私や友達がこれをやった。

Or

> これは私や友達がしました。

The limit is actually implicit in 「と」。 In other words,

> 私と友達がやった。

is already read as an exclusive "and", so you only use 「だけ」 to emphasize the
exclusion:

> これは私と友達が自分たちだけで[成し]{なし}[遂げた]{とげた}。
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We use 1b as the meaning of "Just me and a friend did this".

You said だけ can be attached to nouns but it isn't correct. だけ can be attached
to other parts of speech.

You should distinguish だけ and 一人で. だけ means "only", "just" and 一人で means "by
oneself", "alone".

2b doesn't make sense because you and your friend are two, so you can say
これは私と友達が(で)二人でした.
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To avoid confusion as to whether it's "you and friend(s)" or "you and one
other friend", you may want to phrase it this way:

> これは友達と二人 **だけ** でした。  
>  **Only** a friend and I did this.

Assuming the context of you showing that you worked on this directly, you're
implicitly stating that you're a part of this group because 友達と = 'with a
friend' or 'with friends', but also 二人だけで is explicit about two people, so if
there's you, there's only one other person which is your (one) friend. This
also probably sounds the most natural.
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