Learn Hardcore German: Sag mir bitte, ob du morgen Zeit hast, dann planen wir den Spaziergang. - Please tell me whether you have time tomorrow, then we’ll plan the walk.

Because ob du morgen Zeit hast is a subordinate clause (a dependent clause). In German, subordinate clauses are normally separated from the main clause with a comma. Here, the first main clause is Sag mir bitte, and the subordinate clause is ob du morgen Zeit hast.

What exactly does ob mean here, and why not wenn?

ob introduces an indirect yes/no question, roughly whether/if: Tell me whether you have time tomorrow.
wenn usually means if/when (condition or time). Using wenn here would change the meaning to a condition (and it would typically need a matching main clause like wenn ..., dann ...).

Why is the verb hast at the end in ob du morgen Zeit hast?

Because in subordinate clauses introduced by words like ob, German sends the conjugated verb to the end.
Main clause: Du hast morgen Zeit. (verb in position 2)
Subordinate clause: ..., ob du morgen Zeit hast. (verb at the end)

Why does the second part say dann planen wir and not dann wir planen?

German main clauses follow the V2 rule: the conjugated verb must be in the second position.
Here, dann takes the first position, so the verb planen must come next:

Dann planen wir den Spaziergang.
Not: Dann wir planen ...

Is dann required? What’s its function?

It’s not strictly required, but it’s very natural. dann signals then/after that, showing the sequence: 1) you tell me your availability
2) then we plan the walk

Without dann, it would still be grammatical, but the “step-by-step” feeling is weaker.

Why is there a comma before dann? Is this one sentence or two?

You have two main clauses:

Sag mir bitte, ob du morgen Zeit hast,
dann planen wir den Spaziergang.

A comma can be used to separate main clauses, especially when the second one is closely connected and begins with an adverb like dann. Many writers would also choose:

a semicolon: ..., ob du morgen Zeit hast; dann planen wir den Spaziergang.
or two sentences: ..., ob du morgen Zeit hast. Dann planen wir den Spaziergang.

What form is Sag? Why not Sage?

Sag is the informal singular imperative (command) of sagen addressed to du. In everyday German, Sag! is much more common than Sage! (the longer form exists but sounds more formal/old-fashioned).

Formal (to Sie) would be:

Sagen Sie mir bitte, ob Sie morgen Zeit haben. Dann planen wir den Spaziergang.

Why is it mir and not mich?

Because sagen typically takes:

the person you’re telling = dative (mir, dir, ihm …)
the thing you’re telling = accusative (or a whole clause)

So mir is dative: Tell me ...
Here, the “thing” being told is the whole clause ob du morgen Zeit hast, which functions like the object content.

Does bitte have to be in the middle? Where else can it go?

bitte is flexible. All of these are common, with slightly different emphasis:

Sag mir bitte, ob du morgen Zeit hast. (neutral)
Bitte sag mir, ob du morgen Zeit hast. (more emphasis on politeness/request)
Sag mir, ob du morgen Zeit hast, bitte. (often more casual, sometimes slightly insistent depending on tone)

Why is it Zeit hast without an article? What does Zeit haben mean?

Zeit haben is a very common expression meaning to have time / to be available. In this sense, Zeit is used without an article:

Hast du morgen Zeit? = Are you free tomorrow?

You can use an article in other meanings, e.g. die Zeit as “time (in general)” or a specific time period, but not in the standard “availability” phrase.

Why is it den Spaziergang (with den)?

Because Spaziergang is masculine (der Spaziergang) and it’s the direct object of planen, so it’s in the accusative case:

nominative: der Spaziergang
accusative: den Spaziergang

Using den implies a specific walk you have in mind (e.g., “the walk we talked about” / “our walk”).