The text you provided is a collection of sentences that are not connected to each other. They seem to be fragments of different conversations or texts. Here's a breakdown of each sentence:

1. "I've no idea at all. I suppose it's only understandable to those who wrote it." - This sentence expresses confusion or lack of understanding about something that was written or said.
2. "Milk and other dairy products are very rich in substances necessary for building good health." - This sentence provides information about the nutritional benefits of milk and dairy products.
3. "Are you sure you want to join in? The task that we are going to carry out will be very exhausting and difficult." - This sentence is a question expressing doubt or concern about someone's willingness to participate in a challenging task.
4. "The most serious changes they have introduced have affected education at the elementary level." - This sentence describes significant changes that have impacted education for young children.
5. "I don't envy Mr. Carter in the least. Living such a lonely life as he does must certainly be very painful." - This sentence expresses a lack of sympathy or pity for Mr. Carter's situation and acknowledges the difficulty of his solitary life.
6. "Alex is not too happy with his new class. The youths still consider him a stranger." - This sentence describes Alex's feelings of discomfort or unhappiness in his new class and the fact that he is still seen as an outsider by his classmates.
7. "What a first-class idea! Imagine sailing together across the ocean in a yacht like this." - This sentence expresses excitement and enthusiasm about the idea of sailing together on a yacht.
8. "No wonder they can afford to buy a new house every year; their father earns such a huge amount of money." - This sentence explains the reason for someone's ability to afford expensive purchases, attributing it to their father's high income.
9. "Unfortunately, the sea isn't always so quiet and peaceful on this part of the western coast to the economy." - This sentence describes a problem or challenge related to the weather or conditions in a particular coastal area.