1. Until I missed him very much, I called him.
2. Until I missed him very much, I had called him.
(Which one is grammatical?)
3. That I missed her very much was real.
4. It was real that I missed her so much.
(Are both grammatical?)
5. I was so tired, so I stayed in bed until 10 a.m.
(Does this sentence mean that he slept until 10 a.m.?)
#1 is best. Both clauses use the past tense. #2, no, because you change tenses from past to past perfect.
#3 & #4. I don't really like either of them, but they are grammatical. #4 would be better if "so" was replaced by "very". #3 is very informal and might be okay in a sentence, but "that" is a pronoun with no antecedent here. In #4 it is a conjunction which is used correctly.
"That" has at least three uses:
https://www.bing.com/search?q=that+-+definition&form=EDGNTC&qs=PF&cvid=400eefe58b3f4d1d8e02e075c2a0fe13&pq=that%20-%20definition
#5 is fine. It does not say he slept until 10:00, though. He could have been staring at the ceiling, reading, watching television, or sleeping.
1. Both sentences are technically grammatical, but they have slightly different meanings.
In the first sentence, "Until I missed him very much, I called him," it implies that the action of calling him stopped once you started missing him. It suggests that you used to call him regularly, but once you started missing him a lot, you stopped calling.
In the second sentence, "Until I missed him very much, I had called him," it implies that the action of calling him happened before you started missing him. It suggests that you used to call him regularly in the past, and then at some point, you started missing him a lot. The action of calling happened before the feeling of missing.
2. Both sentences are grammatical, but they have different word orders.
In the first sentence, "That I missed her very much was real," it uses a noun phrase ("That I missed her very much") as the subject of the sentence. It means that the fact or feeling of missing her a lot was real.
In the second sentence, "It was real that I missed her so much," it uses a pronoun ("It") as the subject and describes what "it" refers to later in the sentence. In this case, "it" refers to the fact or feeling of missing her a lot, and the sentence means that this feeling was real.
3. No, the sentence "I was so tired, so I stayed in bed until 10 a.m." does not necessarily mean that the person slept until 10 a.m. It only implies that they stayed in bed until that time. They could have been awake and just resting, or they could have been asleep. The sentence doesn't provide specific information about their sleep state.