I was told by my teacher that I don’t need to include the pronoun if I’m referring to “I” or “we” since it’s implied... I thought the preterit tense is more appropriate for the first sentence because I only played those instruments from 2000 to 2005... correct?

English:
Between 2000 and 2005, I played three different instruments: the piano and the guitar as extracurricular activities and the flute for school. I started to practise karate when I was 10 years old. I stopped when I was 15 years old because I achieved a degree in black belt. Also, I started to bowl, alpine ski, cross country ski, play volleyball and billiards (pool) when I was 10 years old. I stopped playing soccer, touch football and cross country skiing when I was 14 years old. Then, I started to play laser tag and paintball. Also, almost all my friends liked to read Harry Potter books and I did not like them (referring to the books...). My father’s youngest brother, --, got married in 2000. My second eldest sister, --, eloped in 2005. When I was 15 years old, I joined my school’s science club and I dissected a rat and a cat.

Spanish:
Entre los años 2000 y 2005, estuvo practicando tres instrumentos diferentes: el piano y la guitarra como actividades extraescolares, y la flauta en la escuela. Empecé a practicar kárate cuando tenía diez años. Lo dejé cuando tenía quince años porque logré un grado de cinta negra. También comencé al bolos, al esquí de alpino, al esquí de fondo, jugaba al voleibol y al billar cuando tenía diez años. Paré jugar al fútbol, al futbol de tacto y al correr de traviesa cuando catorce años. Después empecé jugar al marro con láser y practicar paintball. También a casi todos mis amigos les gustaba leer los libros de Harry Potter, y a mí no me gustan. El hermano de mi padre quién joven, ___, estuve casado en el año 2000 y era una de la dama de honora. Mi segunda hermana mayor, ___, fugó cuando tenía quince años. El año 2005, me hice miembro del club de ciencia, y disequé una rata y un gato.

In the last sentence, you were supposed to say when I was 15. That would be:

Cuando tenía quince años

Ordinarily you are right about the first sentence, however there is a special meaning of estar in the Preterit = I arrived! However, if you insist on the Preterit, you need the correct for. I played = estuve, but better yet "jugué" = I played, I did play.

A few verbs have special meanings in the Preterit. estar = to arrive/"get", like "llegar", tener = to "grab/taje", like coger/tomar. poder = to "manage/be able to do." querer = to "intend." saber = "to find out." You may not have had that yet. What WERE the directions for this exercise? If I knew your textbook, edition, etc. and I have that book, I could find out what the directions are.

It's true about pronouns but since the "yo" form and the "él/ella/usted" forms are identical in the Imperfect (estaba) of they are used if there could be any confusion.

al bolos = either comencé a jugar a los bolos OR comencé los bolos...
alpine ski, = esquí alpino OR alpestre (these are adjectives)...
cross country = a campo traviesa OR a través del país/campo
voleibol = volibol (spelling)
Paré jugar Paré de jugar (some verbs require a preposition before an infinitive

al correr = the English says skiing not running?
cuando catorce años = cuando tenía...
Después, empecé a jugar...
láser is laser but sorry, I have no idea what "laser tag" is! But "to play tag" is jugar al tócame tú"
I did not like them = a mí no me gustaron (the books did not please)
My father’s youngest brother, = El hermano menor de mi padre...se casó (got married)
era una de la dama de honora. = this doesn't show up in the English, but "una de las primeras madrinas de boda" = one of the maids of honor?
fugó = se fugó con un amante en 2005....(Here the Spanish varies from the English.) Cuando tenía quince años (when I was 15),...

NOTE: When your screen name is simply "Anonymous" I can not tell if you are male or female and that could be bad checking adjectives that refer to you. Of course, when you mentioned "maid" of honor, I knew!

Sra

P.S. I think that's it. Scanning up and down is something my eyes don't handle well any more so feel free to type it all out with corrections, if you wish. Having seen your English once, I now have an idea of what you wanted to say. Very interesting!

Yes, you are correct. In English, when referring to yourself (using "I" or "we"), it is not necessary to include the pronoun if it is clear from the context. The preterit tense is more appropriate for the first sentence because it indicates a completed action in the past. So, the correct sentence would be:

"Between 2000 and 2005, I played three different instruments: the piano and the guitar as extracurricular activities and the flute for school."

By using the preterit tense "played," you convey that you played those instruments specifically during the time period mentioned.

In the same way, you can apply this principle when mentioning other actions in the past throughout your paragraph. Just make sure to use the appropriate tense to indicate when the actions took place.