中世史: the Middle Ages; medieval times / 中性子: a neutron / 中性紙: alkaline paper; archival paper

with a mouth open wide / open wide

空き, 明き, 開き: a space before or after a character, especially a punctuation mark, such as the space after 。 、 」 ） etc. or before 「 （ etc.

紅蓮地獄: the Padma Hell