Learn Farsi: لیوان وسطِ میز است. - The glass is in the middle of the table.

That is the Ezafe! In Farsi, when a spatial preposition like vasat (middle) is followed by a noun like miz (table), they must be linked by this short -e sound. It acts a lot like the English word 'of', so vasat-e miz translates literally to 'middle of table'.

What is the small diagonal line under the last letter of vasat (وسطِ)?

That is the vowel mark called a kasra, representing the -e sound for the Ezafe. In everyday Farsi writing, this little line is almost always left out. We include it in learner texts to help you remember to pronounce it as vasat-e miz rather than just vasat miz.

The translation says 'The glass' and 'the table', but where is the word for 'the'?

Farsi doesn't have a direct equivalent for the definite article 'the'. Nouns like livân (glass) and miz (table) are naturally understood as 'the glass' or 'the table' depending on context. If we wanted to say 'a glass', we would use yek livân.

Why is the verb ast (is) at the very end?

In Farsi, verbs almost always go at the very end of a sentence. Unlike English which uses a Subject-Verb-Object order ('The glass is on the table'), Farsi uses Subject-Object-Verb. So, the literal order here is 'Glass middle-of table is'.