Сhoosing between твоих and своих

Question

When should твоих be used and when should своих be used? What are the reasons for picking one over another?

Answer 1

I think that the difference is the following:

"мой, твой, его (её, их)" denote "belonging to the speaker, the addressee, a third person".

"свой" is used when you want to denote "belonging to the subject of the sentence".

If the subject of the sentence is 1st or 2nd person pronoun, "свой" is preferred. In other cases, "мой", "твой", "его (её, их)" is used. It is not a strict rule, though.

Examples:

Я стираю свои штаны - I am washing my pants. ("мои штаны" is also possible)
Я стираю твои штаны - I am washing your pants. ("свои штаны" is not possible, it means "my pants" in this sentence).

If the subject is a quantification pronoun, than "свой" is the only option.

Каждый стирает свои штаны - Everyone is washing their (own) pants. ("его" is not possible)

Please note that the subject is not always a noun in nominative case, it is a more complex notion. It also includes:

• null subjects of imperative clauses, as in "постирай свои штаны" - "wash your pants"
• dative subjects, as in "тебе пора постирать свои штаны" - "it's time for you to wash your pants"

If the sentence is imperative, "свой" is the only option.

Постирай свои штаны - Wash your pants. ("твои" is not possible, at least for me)

Answer 2

I do believe that Olga gave a succinct and accurate explanation. Just want to add that actually there are cases when свой is completely interchangeable with мой, твой etc.

Let's, once again, refer our Bible )

Возможно синонимическое употребление притяжательных местоимений мой – свой, твой – свой и т.п. Ср.: ... Я предаюсь моим мечтам (Пушкин). – Я не потерплю в своем доме воров (Чехов). Употребление притяжательных местоимений мой, твой, наш, ваш вместо возможного по условиям контекста свой больше подчеркивает связь с соответствующим лицом, в частности при противопоставлении, например: Моей главы коснись твоей рукой (Жуковский).

Д. Э. Розенталь, "Литературное редактирование текста", "Возвратные и притяжательные местоимения".

To translate, this pronouns can be synonymous, especially when we can suspect a logical opposition - Touch my head with your hand.