Sous la pluie, le pelage de la chienne reste humide longtemps, donc je garde une serviette près de la laisse.

Sous la pluie is the normal French way to say in the rain or out in the rain.

• sous literally means under, but with weather it often corresponds to natural English expressions like in the rain
• dans la pluie would sound unusual here, because it suggests being physically inside the rain as a substance rather than using the normal weather expression

So for weather exposure, sous la pluie is the idiomatic choice.

Le pelage means the coat or fur of an animal as a whole.

Compare:

• le pelage = the animal’s coat/fur collectively
• les poils = hairs, or the animal’s hairs/fur in a more individual or general sense

So:

• le pelage de la chienne = the dog’s coat
• les poils de la chienne = the dog’s hairs/fur

In this sentence, pelage sounds a bit more natural and specific for talking about the coat staying wet.

La chienne means the female dog. French marks grammatical gender, and here the animal is specifically female.

• le chien = male dog, or dog in a general sense
• la chienne = female dog

So le pelage de la chienne means the female dog’s coat.

Also, unlike English, chienne is the normal animal term in French. Even though the English equivalent bitch can sound offensive in many contexts, chienne is not automatically rude in French when referring to an actual dog.

Rester means to remain or to stay, so reste humide longtemps means stays wet for a long time.

That is slightly different from est humide, which simply means is wet.

• est humide = is wet
• reste humide = remains/stays wet

Here reste emphasizes duration: after getting wet in the rain, the coat does not dry quickly.

Both can relate to wetness, but they are not exactly the same.

• humide = damp, moist, wet
• mouillé = wet, soaked, made wet

In this sentence, humide works well because it suggests the coat stays damp/wet over time. It can sound a bit more natural when talking about something remaining wet for a while.

If you said reste mouillé longtemps, that would also be understandable, but humide can feel slightly more descriptive and less blunt.

Because humide agrees with pelage, not with chienne.

The subject is:

• le pelage = masculine singular

So the adjective must match pelage:

• le pelage reste humide

Even though chienne is feminine, it is inside the phrase de la chienne and is not the noun being described by humide.

Longtemps is an adverb meaning for a long time, so it modifies the whole idea reste humide.

French commonly places this kind of adverb after the verb phrase:

• reste humide longtemps

This is very natural word order in French.

If you translated too literally from English, you might expect something like longtemps reste humide, but that would not be the normal order here.

Here donc means so, therefore, or as a result.

The logic is:

• the dog’s coat stays wet a long time
• donc = so / therefore
• I keep a towel near the leash

Donc often sounds a bit more direct or logical than alors, though in many everyday contexts they can both connect ideas.

In this sentence, donc clearly introduces a consequence.

Garder here means to keep.

• je garde une serviette près de la laisse = I keep a towel near the leash

Other verbs would change the meaning:

• tenir = to hold, keep in one’s hand, maintain
• laisser = to leave, let

So garder is the best choice for the idea of keeping an item available in a certain place.

Because près is used with de in this expression:

• près de = near, close to

So you must say:

• près de la laisse = near the leash

You cannot say près la laisse in standard French.

Also note the article:

• de la laisse because laisse is feminine singular
• there is no contraction here, because de + la stays de la
• only de + le becomes du, and de + les becomes des

This is about definite vs. indefinite articles.

• une serviette = a towel, any towel, one towel not previously specified
• la laisse = the leash, a specific leash known in the situation

The leash is likely understood as the dog’s usual leash, so French naturally uses the definite article.

This is very common in French: a familiar, identifiable object often takes le/la/les, even where English might still say a in some contexts.

The commas help readability, but the meaning would still be clear without them.

• Sous la pluie, sets off the introductory phrase
• the comma before donc helps mark the result/consequence

So the commas are natural and helpful:

• Sous la pluie, le pelage de la chienne reste humide longtemps, donc je garde une serviette près de la laisse.

You may sometimes see slightly different punctuation in informal writing, but this version is well-structured and clear.