I still have a few other sentences for you to check. Then, I didn't understand one of your corrections. Thank you for helping me.

1) Why shall I capitalize "the" in the sentence:
To the left and to the right of the mannequin respectively there is a sewing machine.
2) When I'm at the seaside, I like looking at the blue sky when it gets dark and enjoying (?) the cool breeze and listening to the sound of the waves or the cry of a seagull.
3) I also lik relaxing ona bed-chair or reading a good book or even taking a nap and sleeping in the warm sun.
4) I also like helping my child making suncastles or filling small moulds with sand.
5) When I'm in the sea, I like splashing my friends, diving into the water and swimming breasttroke to the buoy (or raft, float??).
6) I prefer listening to music on my MP3 player (rather??) than on my cell phone.

1. There should be no capital T on "the."

2. Smooth out the series: When I'm at the seaside, I like looking at the blue sky when it gets dark, enjoying the cool breeze, and listening to the sound of the waves and the cries of a seagull.

3. like <<spelling; ... relaxing on a chaise lounge, reading a good book, or ...

4. sandcastles

5. comma needed; any of the choices work, depending on what you mean.

6. Yes, include "rather".

Sure, I'd be happy to help you with your sentences. Here are the corrections and explanations:

1) Why should I capitalize "the" in the sentence:
To the left and to the right of the mannequin respectively, there is a sewing machine.
In this sentence, "the" should be capitalized because it is the first word after a colon (:). In English grammar, when a complete sentence follows a colon, the first word of that sentence should be capitalized.

2) When I'm at the seaside, I like looking at the blue sky when it gets dark and enjoying the cool breeze, and listening to the sound of the waves or the cry of a seagull.
This sentence looks fine. I added a comma after "cool breeze" to create a clearer pause between two separate actions.

3) I also like relaxing on a bed-chair or reading a good book, or even taking a nap and sleeping in the warm sun.
This sentence looks fine. I added a comma after "good book" to separate the different activities being mentioned.

4) I also like helping my child make sandcastles or filling small molds with sand.
I changed "making" to "make" because "helping" already implies that the action is being done together with someone else. I also changed "suncastles" to "sandcastles" as it is the correct term. Additionally, I changed "moulds" to "molds" as "molds" is the more commonly used term in American English.

5) When I'm in the sea, I like splashing my friends, diving into the water, and swimming breaststroke to the buoy (or raft, float??).
This sentence looks fine. I added a comma after "water" to separate the different actions being mentioned. As for the word choice between "buoy," "raft," or "float," it depends on the specific object you are referring to. A buoy is a floating marker, a raft is a flat structure typically used for floating on water, and a float is usually an inflatable object used for recreational purposes. Choose the word that best fits the object you have in mind.

6) I prefer listening to music on my MP3 player rather than on my cell phone.
This sentence looks fine. I replaced "than" with "rather than" for clearer emphasis, but either word can be used in this context.

Let me know if you have any more questions or need further assistance!