Das war umsonst.

Das refers to some specific thing, action, effort, or situation that has already been mentioned or is obvious from context.

For example, it could mean:

• that trip
• that attempt
• all that work
• what you just did

So das works a lot like English that in That was for nothing.

German main clauses normally follow the verb-second rule.

In Das war umsonst:

• Das is in the first position
• war is the finite verb, so it must come second
• umsonst comes after that

So the basic structure is:

Das + war + umsonst

This is very normal German word order.

War is the past tense of sein, so the speaker is talking about something that is already over.

• Das war umsonst = the whole thing turned out to be for nothing
• Das ist umsonst would usually describe something in the present

Be careful, though: Das ist umsonst can also mean That is free of charge, depending on context. With war, the sentence often sounds like someone is looking back and saying the effort was wasted.

Here, umsonst is used as an adverb or predicative expression.

It does not change its form:

• not umsonste
• not umsonster

So you simply say:

• Das war umsonst
• Alles war umsonst
• Die Mühe war umsonst

Even though English might use an adjective-like expression such as useless or in vain, German uses umsonst unchanged.

Yes. That is one of the most important things to know about umsonst.

It can mean either:

• in vain / for nothing
• free / free of charge

Examples:

• Ich habe umsonst gewartet. = I waited in vain.
• Das ist umsonst. = That is free.

In Das war umsonst, context often points to for nothing / in vain, but in some situations it could also mean That was free. Context tells you which meaning is intended.

They are very close, but not perfectly identical.

• umsonst is common and everyday
• vergeblich is a bit more formal and clearly means in vain

So:

• Das war umsonst = natural, everyday German
• Das war vergeblich = also correct, but slightly more formal and less ambiguous

A useful difference is that vergeblich cannot mean free of charge, so it is sometimes clearer.

Yes, you can.

The difference is mainly one of focus:

• Das war umsonst points more directly to a specific thing: that
• Es war umsonst can sound a bit more general: it was for nothing

In many situations, both are possible. But if you are reacting to something concrete that was just mentioned or seen, das often feels especially natural.

It is pronounced roughly like oom-ZONST.

A few helpful points:

• the stress is on sonst
• the s in sonst sounds like z in English
• the o is short
• the final st is a clear st sound

So a simple English-friendly guide is:

umsonst → oom-ZONST

Das war umsonst is completely normal, neutral German. It works well in everyday speech and writing.

It is not slang, and it is not especially formal either. You could use it:

• in conversation
• in a story
• in a complaint
• in a serious situation

If you wanted something a bit more formal or literary, vergeblich might be more likely:

• Alle Bemühungen waren vergeblich.

But for ordinary speech, Das war umsonst is very natural.