It is hard to comment without seeing the specific sentences. In a nutshell, "the" is used before nouns that refer to a specific instance of something that the reader or listener already knows about, or that is defined within the immediate context. "I saw a cat" (any cat) "I saw the cat" (the cat that you already know about) "I saw the cat from next door" (I'm telling you now which cat it was) With certain idiomatic and special-case exceptions, all singular countable nouns need an article or other determiner.
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AnonymousWhen do I have to put a 'the' before a word and when not?"the" is used to show that the speaker is certain that he has specified exactly which entity in the real world he is referring to — has given enough information for the listener to know which entity is intended.