A Bicycle (2)

After I learned how to ride a bicycle,

I was practicing a lot.

You don't need a new sentence for this.

And one day there was a ditch with nettle in it and a thin wood board lying above.

Nettle is uncountable, no worries.

The height wasn't so high.

Or: It wasn't that high up.

After that I have never fallen on the nettle again.

Irregular past participle.

A few questions though.

Nettle is an uncountable noun right?

Yes.

It means if I use the article 'the' with the word nettle would it be a mistake?

You can use "the" with uncountable nouns - the milk, the coffee, the air. You just can't you "a/an".

And one day there was a ditch with nettle in it and a thin wood board was lying above.

You must use "a" because this is the first time that you are telling us about "thin wood board".

For example:
I saw a thin wood board lying above.
The wood board was about 4 centimeters thick (now we know about "wood board" so it's okay to say "the")

I hope that helps!

It wasn't very high.

It means if I use the article 'the' with the word nettle would it be a mistake?

It's uncountable like water.
Examples: I'd like some water (can't use a/an).
There is water in the bucket (can't use a/an).

The water in the bucket is dirty (it's okay to use "the" to refer to a specific instance of water)
The water that I drank was refreshing.