Learn Hardcore German: Alles, was du gesagt hast, war nett und hilfreich. - Everything that you said was nice and helpful.

Because the grammatical subject is Alles, which is a neuter singular pronoun (“everything”). Verbs agree with the head noun/pronoun, not with the idea of “many things,” so you use the 3rd person singular: war, not waren.

Singular: Alles war gut.
Plural: Alle Dinge waren gut.

Why is the relative pronoun was used after Alles (and not der/die/das)?

In German, when the antecedent is an indefinite like alles, nichts, etwas, vieles, manches, the correct relative pronoun is was, not der/die/das. So you say Alles, was …, Nichts, was …, etc.

Example: Alles, was du gesagt hast, …
Contrast with a definite noun: Die Dinge, die du gesagt hast, …

Can I say Alles, das du gesagt hast?

Not in standard modern German. The normal, idiomatic form is Alles, was du gesagt hast. You may encounter alles, das in older texts or nonstandard usage, but learners should stick to was here.

What case is was in here?

Accusative. Inside the relative clause, was functions as the direct object of gesagt (haben):

Paraphrase: du hast es gesagt → the es corresponds to was. Note: was looks the same in nominative and accusative; context (the verb’s valency) tells you the case. If was were the subject, you’d get something like: Alles, was hilft, ist gut.

Why is the word order gesagt hast (not hast gesagt)?

Because was du gesagt hast is a subordinate (relative) clause. In subordinate clauses, the finite verb goes to the end. In the perfect tense, the participle precedes the auxiliary, so you get the cluster gesagt hast at the end of the clause.

Main clause: Du hast gesagt …
Subordinate clause: …, dass du gesagt hast … / …, was du gesagt hast, …

Do both commas have to be there?

Yes. German requires commas to set off subordinate clauses. The relative clause was du gesagt hast is enclosed by commas: one before it and one after it:

Alles, was du gesagt hast, war …

Why use the perfect (hast gesagt) instead of the simple past (sagtest)?

Spoken German prefers the perfect for most verbs in past narration. Alles, was du sagtest, … is grammatically correct and common in formal writing and in some regions (especially the North), but in everyday speech hast gesagt sounds more natural.

Could I use present ist instead of past war?

Yes, if you want to stress a current, still-valid evaluation: Alles, was du gesagt hast, ist nett und hilfreich. Using war frames it as a past assessment (e.g., about a conversation that already concluded). Both are possible; choose based on time reference and nuance.

Why don’t nett and hilfreich have adjective endings?

They’re predicate adjectives after sein and therefore appear in their base form (no endings). Adjective endings are used when the adjective directly modifies a noun (attributive position):

Predicate: … war nett und hilfreich.
Attributive: deine netten und hilfreichen Tipps

Is Alles capitalized because it’s a noun?

No. Alles here is an indefinite pronoun. It’s capitalized only because it’s the first word of the sentence. In the middle of a sentence, it would be lowercase: … weil alles gut war.

How would this change with different forms of “you” (formal or plural)?

Informal singular (du): Alles, was du gesagt hast, war …
Informal plural (ihr): Alles, was ihr gesagt habt, war …
Formal (Sie), singular or plural: Alles, was Sie gesagt haben, war …

What’s the difference between Alles, was du gesagt hast and Was du gesagt hast?

Alles, was du gesagt hast emphasizes the totality: “everything that you said (all the individual points).”
Was du gesagt hast treats it as one unit: “what you said” as a whole. Both can work, but the nuance is slightly different.

Could I rephrase the sentence another way?

Yes. Natural alternatives include:

Es war nett und hilfreich, was du gesagt hast. (Expletive “es” as subject)
Alles, was du sagtest, war nett und hilfreich. (simple past; more formal/region-dependent)
Was du gesagt hast, war nett und hilfreich. (drops “alles,” changes the nuance as above)

Why is the auxiliary haben used (not sein) in hast gesagt?

Most transitive verbs (like sagen) form the perfect with haben. Sein is reserved for a limited set of intransitives that involve movement or change of state (e.g., gehen → ist gegangen, sterben → ist gestorben) and for sein/bleiben/werden themselves.

When would waren be correct here?

When the grammatical subject is plural:

Die Punkte, die du genannt hast, waren nett und hilfreich.
Alle Dinge, die du gesagt hast, waren … (grammatically fine but less idiomatic than the version with Alles in this context)