The definite article the is pronounce / ði / before vowels and mute h.
Before all other sounds it is pronounced / ðə /.

The Definite Article
The / ði /:
the artist | /ði ˈɑːtɪst/ |
the apple | /ði ˈap(ə)l / |
the egg | /ði ˈɛɡ / |
the end | /ði ˈɛnd/ |
the hour | /ði ˈaʊə/ |
the honor | /ði ˈɒnə/ |
The / ðə /:
the boy | / ðə bɔɪ / |
the street | / ðə striːt / |
the park | /ðə pa:k / |
the picture | /ðə pɪktʃə / |
the dog | /ðə dɒɡ/ |
The definite article has the same form for the singular and the plural – the.
the mountain | the mountains |
the answer | the answers |
the car | the cars |
the park | the parks |
the town | the towns |
the dog | the dogs |