I wonder
Informal Form +
Verb
Verbs in their informal forms (present or past) can be followed directly by
雨が降る
I wonder if it will rain.
彼は食べた
I wonder if he ate.
い-Adjective
い-Adjectives in their informal forms (present or past) can be followed directly by
高い
I wonder if it's expensive.
寒かった
I wonder if it was cold.
な-Adjective
For な-Adjectives, the dictionary form (without だ) is used for the present tense. For the past tense, the informal past form is used.
彼女は暇
I wonder if she is free.
静かだった
I wonder if it was quiet.
Noun
For Nouns, the noun itself is used for the present tense (omitting だ). For the past tense, the informal past form is used.
あれは猫
I wonder if that is a cat.
日曜日だった
I wonder if it was a Sunday.