В изолированном виде THE представлят собой слово оканчивающееся на гласный звук, что соответствует открытому типу слога, на который распространяется правило: в открытом слоге корневые гласные произносятся как в алфавите. В данном случае в THE буква “E” является одновременно и корневой, и финальной т.е. она сама открывает слог и сама заставляет себя произносить как в алфавите.

В случае фразы, правило открытого слога соблюдается также:

the hotel [ðə]
the onion [ði]
the hour [ði]
the University [ðə]
the one [ðə]
the only [ði]

the union
the apple
the inn
the once
the ounce

A man was trying to build an electric motor-car. He worked in an office in the town during most of the week, but on Saturdays and Sundays he stayed at home in the country and worked on the electric car. One Monday morning he came to the office and said to his friends, ‘I have done it! I have driven from my home to here by electricity!’
His friends were all very glad. ‘How much did it cost to get here by electricity?’ they asked.
‘Three hundred and two pounds,’ he answered. ‘Two pounds for the electricity, and three hundred pounds for the electric wires from my house to the amazing car.’

А в таких сочетаниях с учетом полуотмеченности структур:

the one
the yellow
the road
the fire
the tree
the brown
the answer

the one [ðiə]
the yellow [ðiə]
the road [ðiə]
the fire [ðə]
the tree [ðə]
the brown [ðəi]
the answer [ði:]