It's not really a question of whether 'there' is correct or incorrect, it's more a case that no one seems to understand what the sentence is supposed to mean. Maybe you could write it in a different way, and then we might be able to help more.

The sentence "If there was a case, they would go on a trip" is grammatically incorrect because it uses "there was" in a conditional sentence, where the verb tense should reflect the hypothetical or unreal nature of the situation. In English, when expressing hypothetical situations or things that are not true in the present or future (often using "if" clauses), we typically use the past tense form, even though the situation is not actually in the past. In this case, the correct structure for a present unreal condition would be: "If there were a case, they would go on a trip." Here’s why: "If" clauses describing hypothetical or unreal situations (especially when talking about present or future possibilities) typically use the past subjunctive. In this case, "were" is the correct form (subjunctive mood) instead of "was," even though we are talking about something in the present. "There" is used in the structure of sentences like "There is" or "There are" to indicate the existence of something. However, in the context of a hypothetical situation (a condition that is not real), we use "were" instead of "was" to reflect that hypothetical nature. So, the correct version of the sentence is: "If there were a case, they would go on a trip." This shows that the situation is hypothetical and not based on real circumstances.

maybe this is a context specific sentence, but for me as a native English speaker, it doesn't fully make sense as it's connecting two seemingly unrelated concepts. The first is a case meaning usually a legal / court case, and the second being a trip as in traveling, so if there's not more context it's really hard to understand what it's trying to mean. It's not really an issue with "there" being used it's just the sentence is confusing.

A "case" as in a legal or court case? It's not wrong necessarily, just awkwardly phrased. It's better to use the subjunctive "were." "If there were a case happening that day, the lawyers wouldn't go on a trip."

Yes. English can get confusing because one word can sometimes have many meanings!