Idioms you must learn whole
Fixed expressions behave like single vocabulary items: their verb bends into any tense, while every other word is locked and no synonym may be substituted.
He let the cat out of the bag before the party had even started.
A fixed expression works like a single item of vocabulary, not like a sentence you assemble word by word. Only the verb carrying the tense is free: it inflects for whatever tense and person the context needs, exactly as it would anywhere else, and if the expression leaves an open slot for a person, any person can fill it. Everything else is frozen - the nouns, their articles, any second verb, the prepositions and the singular or plural choice are all part of the stored item and cannot be edited. Replace one of those locked words with a synonym meaning precisely the same thing and the expression stops working; a listener then hears a literal statement instead.
Two habits give a learner away here. The first is reaching for a synonym: in the opening sentence, both the animal and the container it escapes from feel replaceable, since a dictionary offers two or three equivalents for each - but any substitution turns the line literal and the meaning vanishes. The second is adjusting the number: the frozen noun in the third example sentence is plural and stays plural whatever the subject, while the frozen noun in the first example sentence is singular and keeps its article every time. Only the second example sentence has a genuinely open slot, the person left uninformed, and that slot changes freely while the fixed words around it never do.