Not "I were at home." — say "I was at home.". I の過去の形は were ではなく was です。
やってみよう: Yesterday it ___ very cold. (was / were / is) 答え: was — It には were ではなく was を使います。（「Is」は現在形で、「yesterday」から過去だとわかります。）したがって: it was cold。
You / we / they: were
you, we, または they のときは were を使います。
You were late.
We were happy.
They were at school.
Not "They was at school." — say "They were at school.". they のときは were を使います。was は I, he, she, it のときだけです。
やってみよう: Amina and Hiroshi ___ at Magaff Library. (were / was / are) 答え: were — Amina and Hiroshi ＝ they なので、was ではなく were になります: They were at Magaff Library。
was それとも were？
選ぶ前に「だれ？」と考えてみましょう。I/he/she/it → was、you/we/they → were です。
He was here. / They were here.
I was tired. / We were tired.
Not "He were here. / They was here." — say "He was here. / They were here.". 主語によって決まります。1人・1つ（I/he/she/it）は was、you/we/they は were です。
やってみよう: Lars ___ at the park, but his friends ___ at the gym. (was … were / were … was / was … was) 答え: was … were — Lars ＝ he → was。His friends ＝ they → were。だれかによってそれぞれ決まります。
否定文: wasn't / weren't
was や were の後ろに not を足します: wasn't（I/he/she/it）、weren't（you/we/they）。
I wasn't hungry.
He wasn't there.
They weren't happy.
Not "I not was hungry." — say "I wasn't hungry.". not は was の前ではなく後ろにつきます。
やってみよう: She ___ at home last night. (wasn't / weren't / not was) 答え: wasn't — She には was を使うので、否定は（weren't ではなく）wasn't になり、not は後ろにつきます: She wasn't at home。
疑問文: Was / Were を文頭に
質問するときは、主語の前に Was または Were を置きます。
Was she at home?
Were you late?
Was it cold?
Not "You were late? (asking)" — say "Were you late?". 疑問文では、were が最初に来てそのあとに主語が続きます。
やってみよう: ___ you late for class? (Were / Was / Did) 答え: Were — You には were を使い、疑問文では文頭に置きます: Were you late?（Was は I/he/she/it に使います。）
やってみよう: Remember: who is it? Mei ___ at the cooking class. (was / were / are) 答え: was — Mei ＝ she → was。（これが基本の選択です: I/he/she/it → was、you/we/they → were。）