from what should I know that it is "the" and not "a"; / The more complete a noun is, the earlier it appears in a sentence with the verb olla, "to be". As a result, a noun or a noun phrase that starts a sentence is usually translated with a definite article. If the sentence has another noun with another function, that noun is less complete. It can be translated with both types of articles found in English, depending on the context. The indefinite article is the more likely option in most cases. /
"The large yard" should be accepted. Essentially, big and large mean exactly the same. "Big" can be used more widely and in more senses, but in this case, big and large mean exactly the same. Even dictionaries use these words to define each other. I can see how you are trying to differentiate between "iso" and "suuri", but even those were mixed up in another sentence when it came to translation. I don't know if there is a significant difference between the 2 in Finnish, perhaps, a native speaker can explain, but in English there is no difference when it comes to describing the size of a yard or whatever else, for that matter.
To you maybe, but there are certain rules for the rest of us. :) And the choice of partitive and genitive depends on whether the action is an ongoing process or not. Partitive is used when an action is a process, with numerals, when there is negation, and sometimes to indicate doubt when using a question.