〜ませんでした

1. Origins, structure, and basic meaning

• Structure: Change the polite verb negative ません to ませんでした.
• Basic usage: It states that an action did not happen in the past, as in 昨日はテレビを見ませんでした。.

2. Common pitfalls and similar expressions

• Plain and polite style
  • 見なかった and 見ませんでした express the same past negation in different styles. The polite form uses the stem for ます.
• Not doing and being unable to do
  • 眠りませんでした says someone did not sleep; 眠れませんでした says they could not sleep. In 昨夜はあまり眠れませんでした。, also account for the potential form and the limited degree.