Learn Hardcore Japanese: watasi ha kutu wo nuida mama heya ni hairimasita.

Here まま expresses that the state resulting from the first action (taking off one’s shoes) remains unchanged while the second action (entering the room) occurs. In other words, “entering the room with one’s shoes still off.”

Why is 脱いだ in the plain past form and not in the て-form or polite form?

In the 〜たまま construction, the verb immediately before まま must be in the plain past (dictionary) form 〜た. This indicates “in the state after X has occurred.” You cannot use the て-form (脱いでまま) or a polite form (脱ぎましたまま).

Can we use other tenses or forms with まま, such as 〜てまま or 〜ましたまま?

No. The only standard pattern is Vたまま. Neither Vてまま nor polite-past Vましたまま is grammatical for expressing a continued state.

Why is the direct-object particle を used with 靴?

Because 脱ぐ (“take off [clothes, shoes, etc.]”) is a transitive verb that requires a direct object. 靴を脱ぐ = “take off shoes.”

Why is に used with 部屋 before 入りました?

Because 入る (“enter”) is an intransitive verb that takes the location being entered as Xに入る. Thus 部屋に入る = “enter the room.”

Why is は placed after 私 and not after 靴?

The particle は marks the topic of the sentence. Here the speaker (“I”) is the topic, so 私 is marked with は. The shoes are simply the object of “take off,” so they use を instead.

Could we omit 私は since it’s obvious who did the action?

Yes. In casual speech or when context makes it clear, you can drop 私は. You’d say 靴を脱いだまま部屋に入りました or even shorter 脱いだまま部屋に入った.

What nuance does this sentence carry?

It emphasizes that the speaker entered the room while still in the state of having removed their shoes—perhaps unexpectedly or by mistake. There’s an implication that entering without shoes is noteworthy or contrary to expectation (e.g., there’s a rule to wear slippers).