1. Logic is interesting because it is fun.
We ordinarily would not read this as an argument but just as an explanation. We are not trying to prove that logic is interesting but giving an answer to the question about why it should be.
2. Logic must not be easy, because it is not fun.
This is clearly intended as an argument (using "must" makes the conclusion stand out, and the comma helps remove any ambiguity). It is not valid since we can see that the conclusion can still be wrong even if the premise is right.
3. If logic is easy then it is interesting.
This is not an argument but simply a single statement expressing a conditional relationship between logic being easy and it being fun. (Be careful not to read "if" as though it is the same as "because.")
4. If logic is easy then it is fun. Logic is not easy, so it must not be fun.
This is definitely an argument. However, it is not valid. Instead it is an example of a very common formal fallacy ("denying the antecedent").
5. If logic is easy then it is fun. Logic is not fun, so it must not be easy.
This is an argument and, yes, it is deductively valid.
6. All students are ambitious, but some students are lazy.
We have two separate statements but no reason to see one as supporting the other in the way a premise supports a conclusion. There is no argument.
7. Only someone ambitious will succeed, so some students will succeed.
This is meant to be read as an argument but there is not yet the kind of pattern that makes the conclusion necessarily true if the premise is true. In fact, if we understood an implied premise to be "some students are ambitious" we would have a faulty pattern (another formal fallacy).
8. No one who is lazy is successful, but no honor students are lazy, so all honor students will be successful.
An argument, but not a valid one. A quick rule is that nothing follows from two negative statements.
9. Today is Thursday, therefore today is Thursday.
Technically this is an argument and it would have to be seen as valid since with a true premise you could not have a false conclusion.
10. Alice is lazy but Alice is not lazy, so Alice will be a success.
Nonsensical as it seems, again technically we have a valid argument since with one premise having to be false the argument does not meet the condition set out to call it invalid (all true premises but a false conclusion).
