Ich stelle die Gießkanne griffbereit auf den Balkon, damit ich die Blumen später nicht vergesse.

German often uses different placement verbs depending on how something is positioned.

• stellen = to place something so that it stands upright
• legen = to lay something down flat
• setzen = to set something into a sitting position

A watering can is normally imagined as standing upright, so stellen is the natural choice:

• Ich stelle die Gießkanne ... = I place the watering can ...

Compare:

• Ich lege das Buch auf den Tisch. = I lay the book on the table.
• Ich setze das Kind auf den Stuhl. = I seat the child on the chair.

Die Gießkanne is the direct object of stelle, so it is in the accusative case.

In this sentence:

• Ich = subject
• stelle = verb
• die Gießkanne = thing being placed

For feminine singular nouns, the article die is the same in both nominative and accusative, so the form itself does not change. You identify the case from the sentence structure:

• Ich is doing the action
• die Gießkanne is receiving the action

So even though die looks the same, it is accusative here.

Griffbereit means ready to hand, within easy reach, or handy.

Grammatically, it is an adjective, but here it is being used adverbially/predicatively, not directly in front of a noun. That is why it does not take an adjective ending.

Compare:

• die griffbereite Gießkanne = the watering can that is within easy reach
  
  • here it is directly before the noun, so it gets an ending
• Ich stelle die Gießkanne griffbereit auf den Balkon.
  
  • here it describes how/in what state the watering can is placed, so no ending

This kind of adjective use is very common in German.

Because auf is a two-way preposition:

• accusative for direction/movement toward a destination
• dative for location

Here, the speaker is putting the watering can onto the balcony, so there is movement toward a place:

• auf den Balkon = onto/to the balcony

If the sentence were just describing where it already is, you would use dative:

• Die Gießkanne steht auf dem Balkon. = The watering can is on the balcony.

So:

• stellen ... auf den Balkon = placing it there
• stehen ... auf dem Balkon = being there

Here damit means so that and introduces a purpose clause.

The sentence structure is:

• Ich stelle die Gießkanne griffbereit auf den Balkon
  = main action
• damit ich die Blumen später nicht vergesse
  = purpose: why I am doing that

So the idea is: I put the watering can there so that I won’t forget the flowers later.

This is one of the most common uses of damit.

Because damit introduces a subordinate clause, and in German subordinate clauses the finite verb goes to the end.

Main clause:

• Ich stelle die Gießkanne ...

Subordinate clause:

• damit ich die Blumen später nicht vergesse

That final position of vergesse is completely normal after conjunctions like:

• dass
• weil
• wenn
• damit

So if you see damit, you should expect the conjugated verb to come at the end of its clause.

Here nicht is negating the verbal idea: not forget.

In a subordinate clause with the verb at the end, nicht often comes before the final verb when it negates the whole predicate or most of the clause:

• damit ich die Blumen später nicht vergesse

This is natural German word order.

If you wanted to negate a specific element instead, the position could change. For example:

• damit ich nicht die Blumen vergesse
  = so that it is not the flowers that I forget

That sounds contrastive, as if you might forget something else instead.

So in your sentence, nicht vergesse simply means do not forget.

Die Blumen is also in the accusative case, because it is the direct object of vergesse.

You are forgetting the flowers, so the flowers receive the action of the verb.

With plural nouns, the article die is the same in nominative and accusative, so again the form does not change. You identify the case from the role in the sentence.

• ich = subject
• vergesse = verb
• die Blumen = direct object

So:

• die Blumen = accusative plural

Yes. Since the subject is the same in both parts of the sentence (ich), German can also use um ... zu:

• Ich stelle die Gießkanne griffbereit auf den Balkon, um die Blumen später nicht zu vergessen.

That is also correct and natural.

A useful rule:

• use um ... zu when the subject of both clauses is the same
• use damit when the subjects are different, or when you want a full clause

So:

• ..., damit ich ... nicht vergesse = full purpose clause
• ..., um ... nicht zu vergessen = more compact infinitive construction

Both work here.

Später is a time adverb meaning later. It tells you when the forgetting might happen.

Its position is somewhat flexible, but some placements sound more natural than others.

Your sentence:

• damit ich die Blumen später nicht vergesse

This is perfectly natural.

You could also say:

• damit ich später die Blumen nicht vergesse

That is also possible.

But the original version is smoother because später sits close to the idea it modifies: the later forgetting.

German adverbs are often movable, but word order affects emphasis and naturalness.

Because German uses a comma to separate a main clause from a subordinate clause.

Since damit ich die Blumen später nicht vergesse is a subordinate clause, it must be set off with a comma:

• Ich stelle die Gießkanne griffbereit auf den Balkon, damit ich die Blumen später nicht vergesse.

This comma is not optional in standard German.

Two different German features are showing up here:

1. Gießkanne is a compound noun

German very often combines nouns into a single word:

• gießen = to water / to pour
• Kanne = can, jug

Together:

• Gießkanne = watering can

English often writes such things as two words or with a space, but German usually makes one compound word.

2. Nouns are capitalized in German

All nouns are capitalized, not just proper names. So in your sentence:

• Gießkanne
• Balkon
• Blumen

are capitalized because they are nouns.

This is completely normal in German and helps you spot nouns quickly when reading.