"The boy has an apple."
Translation:לילד יש תפוח.
When using the possessives יש/אין to mean "to have", you need to add the preposition -ל before the owner. It makes sense if you remember that without this preposition יש means "there is".
There is an apple = יש תפוח
The boy has an apple = לילד יש תפוח (literally, something like "To the boy there is an apple").
Yes, the word you place at the start of the sentence gets more emphasis.
So לילד יש תפוח is like "the boy, has an apple".
You would say לילד יש תפוח, אבל לילדה יש תפוז "the boy has an apple, but the girl has an orange" (in English we can represent emphasis by changing the intonation, and in writing, using italics).
Likewise, you would say:
אין להם אוכל, אבל לי יש
They don't have food, but I do.
In both cases, you need the audio in these contextless sentences to determine whether the definite article is being used. If the prefix is "la" it's definite. If it's "l'" or "li" (a grammatical formation for ease of pronunciation which happens when the main word's first vowel is shewa), then it is indefinite.
So if it's "layeled" - it's "the boy" - and if it's "Lig'varim" it's "men", not "the men"