Here is your paragraph written normally, with all the articles in italics.
The highest mountain in Great Britain is the Ben Nevis, which is situated in the Grampians (a mountain range).
The Pennines are the mountains situated in the central part of Great Britain. They are separated from the Southern Uplands by the valley of River Tyne. The Tyne flows into the North Sea.
The mountain range called the Cambrians is in Wales. The highest mountain in the Cambrians is the Snowdon.
The longest river in Great Britain is the Severn. It rises in the Cambrians and flows into the Bristol Channel. The River Thames is not as long as the Severn but it is the most important river in Great Britain. London, the capital of the United Kingdom, is situated on the Thames.
Next time, please use an image to text generator and paste that into your lesson, so your answerer doesn't have to do that or retype it or anything.