Learn Hardcore German: Ich habe heute kaum Zeit, deshalb lese ich nur eine Seite. - I hardly have time today, therefore I read only one page.

Because Zeit haben is the standard expression for “to have time.” The verb is haben, not sein.
Also, Zeit is the direct object here, so it’s in the accusative: kaum Zeit.

What exactly does kaum mean here? Is it the same as nicht or wenig?

Kaum means “hardly / barely,” i.e., almost no time.
It’s stronger than wenig (“little”) and different from nicht (“not”), which would negate the existence of time entirely.
So kaum Zeit ≈ “barely any time.”

Why is heute placed before kaum Zeit? Could I say Ich habe kaum Zeit heute?

Yes, Ich habe kaum Zeit heute is possible, but Ich habe heute kaum Zeit is more natural because time expressions like heute often come earlier in the sentence.
A common guideline is TEKAMOLO (Time–Cause–Manner–Place), so heute (time) tends to appear before other elements like the object kaum Zeit.

Why is there a comma before deshalb?

Because you’re joining two main clauses.

Ich habe heute kaum Zeit = main clause
deshalb lese ich nur eine Seite = main clause
A comma is standard when you place two independent clauses next to each other like this.

What is the difference between deshalb, darum, and deswegen?

They all mean “therefore / that’s why,” and are often interchangeable.
Differences are mostly style and emphasis:

deshalb: very common, neutral
deswegen: also very common, often a bit more conversational
darum: common, can sound slightly more “pointing back” to what was said
In this sentence, any of the three would work.

Why is the word order deshalb lese ich... and not deshalb ich lese...?

Because deshalb is functioning as a sentence adverb at the start of a main clause, and German main clauses require verb-second (V2) word order.
So:

Position 1: deshalb
Position 2: lese (the verb)
Then: ich
That’s why deshalb ich lese... is incorrect in standard German.

Could I also say ..., deshalb ich nur eine Seite lese?

No, not as a normal main clause. With deshalb you still need V2 word order: deshalb lese ich ...
If you want the verb at the end, you’d need a different structure, e.g. a subordinate clause:

..., weil ich heute kaum Zeit habe, lese ich nur eine Seite.
or
..., weil ich heute kaum Zeit habe, lese ich deshalb nur eine Seite. (less common, but possible)

Why does the verb change from habe to lese—what tense is this?

Both verbs are in the present tense:

ich habe = “I have” (present)
ich lese = “I read / I’m reading” (present)
German present tense often covers both English “I read” and “I am reading,” depending on context.

What does nur do in nur eine Seite?

nur means “only/just” and limits the quantity: you will read only one page, not more.
It typically comes right before what it limits: nur eine Seite.

Why is it eine Seite and not einen Seite or ein Seite?

Seite is a feminine noun: die Seite.
In the accusative singular, feminine articles stay die/eine (they don’t change like masculine ones do).
So: eine Seite is correct.

Is Seite definitely “page” here? Could it mean “side”?

Yes, Seite can mean both “page” and “side,” but with the verb lesen (to read), it strongly implies “page.”
If you were talking about a physical side (left/right), you’d usually have context like linke/rechte Seite.

Could I swap the order and say: Deshalb lese ich heute nur eine Seite?

Yes. That’s also natural.
Both are correct; it’s mainly a question of emphasis:

... deshalb lese ich nur eine Seite. (focus on “only one page”)
... deshalb lese ich heute nur eine Seite. (adds “today” to the second clause; could feel slightly more contrastive: today specifically)

Is it okay to drop ich in the second clause since it’s already obvious?

In standard German, you normally keep the subject: deshalb lese ich ...
Dropping ich is not grammatical in standard written German. In very informal speech, people may shorten sentences, but that’s not a normal rule you should rely on.

How would the sentence change if deshalb introduced a subordinate clause like weil does?

deshalb does not create a subordinate clause; it keeps main-clause word order (V2).
If you use weil (“because”), the verb goes to the end in the subordinate clause:

Ich lese nur eine Seite, weil ich heute kaum Zeit habe.
Or with the cause first:
Weil ich heute kaum Zeit habe, lese ich nur eine Seite.