Degree Of Certainty

This is a "Very Certain" expression.
First, there are two sentences which state the present. Again, we can anticipate that we are familiar with Joe´s Friday routine and the likelihood of him maintaining his routine on this Friday. We use the modal "must" to express a strong certainty about where Joe is.

This is a ·"Very Certain" expression.
This is in fact a two part sentence. After the connector "when" we have a fact about the past - "you told her the news." We know this actually happened. What we don´t know is how she reacted. Depending on the type of news, one reacts differently. The modal used is "must" which carries a strong intention, or certainty. Therefore, we can speculate that it was bad news.

This is a Less Certain expression. The first sentence establishes the result of what the dog did in the past - "The dog is really dirty." However, we don´t know how he got this way. We speculate in a less certain manner because there are many ways in which the dog could have gotten dirty.

This is a "Less Certain" expression.
We don´t have any additional information that compliments the idea, so we need to rely on the modal used. "Might" carries a notion of not being very sure, or certain, about something.

This is a "Very Certain" expression.
The fact about the present is that "they are not answering the phone". This means you are highly sure that they are not home. You can never be 100% sure. Still, because of a given circumstance, it is highly possible that "they" are not home. One expresses this high degree of certainty by using a strong intention with the modal "must".

This is a "Very Certain" expression.
The first sentence expresses a reality in the present. From this we speculate, with a high degree of certainty, about them not returning. The modal used is "can´t"

This is a "Less Certain" expression.
The first sentence defines the kind of speculation one should make. The fact about the present is that "I don´t know . . ." For this reason, one speculates about the past with little or no certainty. The modal used is "may".

This is a "Very Certain expression.
There is no complementary information so we need to rely only the the modal presentation. The modal used is "can´t". When used in combination with "have" (can´t have + verb), it expresses a very certain idea but in a negative way.

This is a "Very Certain" expression.
The first sentence establishes the present reality, which is the current time. To establish some context, consider that Tom is your friend and you are familiar with with usual schedule. You know what he normally does around six o´clock. From this background information, one speculates with a high degree of certainty.
The modal used is "should", but not in the sense that one is suggesting what Tom do. The idea is express as a suggestion to what Tom normally does around this time, and it is also interpreted with a high level of certainty.

This is a "Very Certain" expression.
The modal used is "could" and it is expression the possibility of one being able to do something at a specific point in the past. In other words, I was able to cancel the meeting without any problem, but I didn´t because I saw no reason to do so. I express this certainty, of what I was able to do in the past, with the modal "could". Complemented with "have" and the verb, it expresses the full idea.

This is a "Less Certain" expression.
This statement is centered around "why" Mathew is not here. It is difficult to actually know what other people think. For this reason, one speculates with a low degree of certainty what the other person might have thought, and also done.
The modal used is "might".