Wenn ich heute Zeit habe, möchte ich im Garten weiterzeichnen.

Because wenn introduces a subordinate clause. In German subordinate clauses, the conjugated verb usually goes to the end.

So:

• Wenn ich heute Zeit habe = subordinate clause
• habe is the finite verb, so it goes last

Compare:

• Main clause: Ich habe heute Zeit.
• Subordinate clause: ..., wenn ich heute Zeit habe.

German main clauses follow the verb-second rule. The verb must come in the second position.

Here, the whole subordinate clause Wenn ich heute Zeit habe comes first and counts as position 1. That means the verb of the main clause must come next:

• Wenn ich heute Zeit habe, möchte ich ...

Structure:

1. Wenn ich heute Zeit habe
2. möchte
3. ich

This is very common in German when a subordinate clause comes first.

Here, wenn is best understood as if.

It is often used for:

• conditions: if
• repeated situations: whenever / when

In this sentence, it introduces a condition:

• Wenn ich heute Zeit habe ... = If I have time today ...

Important contrasts:

• wenn = if / when
• wann = when? (question word)
• als = when, for a single event in the past

So you would say:

• Wenn ich Zeit habe, ... = If I have time, ...
• Wann hast du Zeit? = When do you have time?
• Als ich jung war, ... = When I was young, ...

möchte means would like to and sounds softer, more polite, and less forceful than will.

• Ich möchte ... = I would like to ...
• Ich will ... = I want to ... / I intend to ...

So:

• Ich möchte im Garten weiterzeichnen sounds natural and gentle
• Ich will im Garten weiterzeichnen sounds stronger, more determined, and sometimes blunt

Grammatically, möchte is historically related to the Konjunktiv II form of mögen, but learners usually first treat it as a very common way to say would like.

weiterzeichnen means to continue drawing or to keep drawing.

It is built from:

• weiter = further / on / continuing
• zeichnen = to draw

So the idea is not just draw, but carry on drawing.

Examples:

• Ich zeichne. = I am drawing.
• Ich zeichne weiter. = I keep drawing / continue drawing.
• Ich möchte weiterzeichnen. = I would like to continue drawing.

Because möchte is a modal-like verb here, and the other verb stays in the infinitive at the end of the clause.

So in the main clause:

• möchte = the conjugated verb
• weiterzeichnen = infinitive, placed at the end

This is the normal pattern in German:

• Ich möchte lesen.
• Ich möchte schlafen.
• Ich möchte im Garten weiterzeichnen.

im is a contraction of in dem.

• in dem → im

So:

• im Garten = in the garden

Also, im Garten uses the dative case because it describes a location where something happens, not movement into a place.

Compare:

• Ich zeichne im Garten. = I am drawing in the garden.
  (location, dative)
• Ich gehe in den Garten. = I am going into the garden.
  (movement/direction, accusative)

Because Zeit haben is a very common fixed expression meaning to have time.

German often uses Zeit without an article in this meaning, just like English usually says have time, not have a time.

So:

• Ich habe Zeit. = I have time.
• Hast du heute Zeit? = Do you have time today?

Using eine Zeit would usually mean something different and would not fit this normal expression.

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

Here:

• Wenn ich heute Zeit habe, = subordinate clause
• möchte ich im Garten weiterzeichnen. = main clause

So the comma is required.

This is one of the most important punctuation rules in German: subordinate clauses are usually set off with commas.

Some parts can move, but the basic grammar must stay the same.

For example, this is also possible:

• Ich möchte heute im Garten weiterzeichnen, wenn ich Zeit habe.

That changes the emphasis a bit, but the meaning stays similar.

Within the sentence, German has some flexibility, especially with time and place expressions, but these rules still matter:

• subordinate clause verb goes to the end
• main clause verb stays in second position
• infinitive stays at the end with möchte

So the original sentence is very natural and standard, even though some reordering is possible.