Correct the following sentences.

He announced that he will take the plane to Dhaka.

= He announced that he would take the plane to Dhaka.

He knew Mr. Khan stopped smoking five years before. = He had known Mr. Khan had stopped smoking for five years.

On arriving in Dhaka, his friends meet him at the station. = When he arrived in Dhaka, his friends met him at the station

He said that the earth moves round the sun.

In reported speech, when the direct speech expresses a universal truth, a scientific fact, or a habitual action, the tense of the verb in the reported clause does not change, regardless of the tense of the reporting verb (e.g., 'said').

Even though the reporting verb "said" is in the past tense, the fact that "the earth moves round the sun" is an eternal truth. Therefore, its tense remains in the simple present tense ('moves') in the indirect speech.