Moral Theologian Father Jone O.F.M.Cap. says:

"Where the tax laws are only penal laws they oblige in conscience only to accept the penalty inflicted for their transgression (Cf. 57). If they are not mere penal laws they oblige in conscience to the payment of the taxes even prior to a judicial sentence."

"Direct taxes are probably not levied by legislation which obliges in conscience prior to a judicial sentence. Customary interpretation and the intention of the lawgiver would seem to render an obligation in conscience highly questionable in our country. Although citizens should be urged to pay their share of the taxes, nevertheless, post factum, it is not necessary to urge restitution."

"Indirect taxes are commonly held to be levied as penal laws."

To my understanding, in practical terms, he is basically saying that you only have to pay what you are forced to pay after a judicial sentence. Anyting else that you can getaway without paying, is no sin.

Thanks, this is what I was looking for.

Do you know what he means by "only penal law", as opposed to criminal law?

I know that all authority is from God.
I know that funding abortion is misuse of authority.

I don't know if not reporting income is immoral.

I agree with the first two. As for the third, I cannot see how not reporting income is immoral.

This quote is from the thread, https://www.cathinfo.com/catholic-living-in-the-modern-world/morality-of-working-for-a-company-that-will-pay-for-employees-abortion/
which I thought might be of interest

I have a friend  who realized in 1979 that his taxes were paying for abortions. We met and became firm friends in the 1990's by which time he had not paid a cent tax for over 10 years and he still does not pay tax up until today.
For those interested in how he did it and what it cost him read here. (It is quite a long docuмent)
http://www.heavenonearthproject.net/page9/files/3cc68aa620842be9e250e8de18a736d6-6.html

I am not a native English speaker, but, in my understanding, penal law and criminal law are both the same thing.

For all practical purposes, this is accurate, at least in the United States.  In regards to income tax, I think the moral obligation would only extend to what is actually income.  The State does, of course, consider that one's income includes anything of any value that one might receive for any reason.  I think this can be legitimately and morally questioned.  The State might punish a person for having a different opinion on the matter, but I don't think that would, in itself, be sinful if one acted upon a different opinion.

For the life of me, I do not understand how a "tip" to a waitress or other servant can be considered income--which is why I always give tips in cash personally to the individual and not part of a docuмented bill.  If the person I give such money to opts to claim it as income, that is his or her decision.