Learn Hardcore Dutch: De grootouder leest een boek in de tuin. - The grandparent reads a book in the garden.

Most nouns referring to people (like grootouder) are de-words.
Compounds ending in -ouder (“parent”) also take de.
You’ll need to learn each noun’s article by practice and exposure. One tip: all plural nouns use de anyway, so once you know the plural, you know the article too.

Is grootouder gender-neutral? How would I say “grandfather” or “grandmother” specifically?

Yes—grootouder simply means “grandparent” without specifying gender. If you want to be explicit:

grootvader or informal opa = grandfather
grootmoeder or informal oma = grandmother

How is the verb lezen conjugated here as leest? Why not something else?

Dutch present-tense conjugation for lezen (to read) goes like this (singular forms):

ik lees
jij leest
hij/zij/het leest
In our sentence the subject is de grootouder (he or she), so you use the third-person singular ending -t, giving leest.

Why does leest have a double e? Does that affect pronunciation?

The double e in leest indicates a long /eː/ vowel, similar to the “ay” sound in English “day,” but shorter. Since the verb stem of lezen is lees-, you keep both e’s when adding the -t ending.

Why is the phrase een boek used instead of het boek?

een = “a/an,” used when you introduce something unspecified or for the first time.
het = “the,” used when you talk about something specific or already known to listener and speaker.
Here we mean “a book,” not one in particular, so een boek is correct.

How do you form the plural of grootouder, boek and tuin?

grootouder → grootouders (add -s)
boek → boeken (add -en)
tuin → tuinen (add -en)
Note: Dutch plurals often take -en, but some words take -s (especially those ending in a vowel or unstressed syllable).

Why is the prepositional phrase in de tuin placed at the end? Could I start with it?

Dutch main clauses follow the V2 (verb-second) rule: one element (subject, object or adverbial) comes first, the conjugated verb comes second, then the rest. In De grootouder leest een boek in de tuin, the subject comes first. You can also front in de tuin as your first element:
“In de tuin leest de grootouder een boek.”
Here in de tuin (the place) is first, leest remains in second position, and you swap the subject and object accordingly.

How do you pronounce tuin? The spelling looks odd.

The Dutch digraph ui represents a unique diphthong [œy] (no direct English equivalent). To approximate: start with an “uh” sound, then glide toward a short “ee.” It takes practice—listen to native speakers or use audio resources.

What’s the structure of the compound grootouder? How do Dutch compounds work?

grootouder = groot (big/great) + ouder (parent). In Dutch compounds, the first element modifies the second, and you write them together without spaces or hyphens. The main stress usually falls on the first part: GROOT-ouder.