Could you please check which tense is best in the following sentences? Thank you. (The poet remembers a walk he went on with his sister, during which he was struck by the song of a Highland Lass.)

1) The poet doesn’t know the song she is singing.
2) As a matter of fact, the poet couldn’t understand the words of the song as the girl was presumably singing in Gaelic, the native language of the Scottish Highlands.
3) He speculates that her song might be about heroic events of the past, about everyday life or a painful experience.
4) Whatever she sings (or sang?) about, he says, he listened “motionless and still,” and as he travelled up the hill, he carried her song with him in his heart long after he could no longer hear it.

1) OK

2) As a matter of fact, the poet cannot understand the words of the song as the girl is presumably singing in Gaelic, the native language of the Scottish Highlands.

3) OK

4) Whatever she is singing about, he says, he listens “motionless and still,” and as he travels up the hill, he carries her song with him in his heart long after he can no longer hear it.

1) The poet doesn’t know the song she is singing.

In this sentence, the present tense "doesn't know" is appropriate because it describes the poet's current state of not knowing the song. To determine this, we can examine the context of the sentence and whether it refers to an ongoing action or a general fact. In this case, it is a general fact that the poet does not know the song, so the present tense is appropriate.

2) As a matter of fact, the poet couldn’t understand the words of the song as the girl was presumably singing in Gaelic, the native language of the Scottish Highlands.

In this sentence, the past tense "couldn't understand" is suitable because it describes the poet's inability to understand the words while the girl was singing. This action happened in the past, so the past tense is appropriate.

3) He speculates that her song might be about heroic events of the past, about everyday life, or a painful experience.

In this sentence, the present tense "speculates" is appropriate because it describes the ongoing action of the poet's speculation. The poet is currently considering the possibilities, so the present tense is appropriate.

4) Whatever she sings (or sang?) about, he says, he listened "motionless and still," and as he traveled up the hill, he carried her song with him in his heart long after he could no longer hear it.

In this sentence, both the present tense "sings" and the past tense "sang" can be considered. It depends on the intended meaning. If the poet means that regardless of what the girl is currently singing about, he listened and carried the song in his heart, then the present tense "sings" is appropriate. However, if the poet means that regardless of what the girl sang about in the past, he listened and carried the song in his heart, then the past tense "sang" is appropriate. The choice between the two tenses depends on the intended meaning and the specific context of the situation.