REMINDER: There is NO requirement to file a tax return for the 2018 tax year until the end of April 2019.
As
@spyfy quoted, in the very first post in this topic, from Canadian government information:
see
https://www.canada.ca/en/immigration-refugees-citizenship/corporate/publications-manuals/operational-bulletins-manuals/canadian-citizenship/grant/residence/income-tax-filing.html
Thus, applicants can check "NO" -- "NO" (not required and did not file) for 2018 until the end of April 2019. No need to clarify or explain. And this should count as one of the three years of required compliance with tax filing obligations. (BUT the applicant should be sure to file the 2018 return timely; see additional observation about this below.)
If possible, however, given that as of earlier this week CRA began accepting 2018 tax returns, the better approach is to file a return and check "YES" -- "YES" (required to file and did file). With rare exceptions, waiting a little while to make the citizenship application in order to do this is simply a good idea for the vast majority of applicants.
The following observation applies to only a few applicants, those who have not filed a 2018 return as yet AND who are relying on 2018 as one of the years they have complied with tax filing obligations:
Note: applicants MUST continue to meet all the requirements for citizenship right up to when the oath is taken. I do not know that processing agents (such as the official conducting the PI Interview) review the tax filing information again attendant preparation for or in conducting the interview, BUT if the applicant (applying in 2019 but before April 30, 2019) was relying on a "NO" -- "NO" response for tax year 2018 counting as ONE of the NECESSARY THREE YEARS of COMPLIANCE . . . THEN, after April 30, 2018, if a tax return is required, that year will technically NO LONGER count as one of the THREE years of compliance unless, of course, a tax return has been filed. Thus, for the applicant who makes an application in 2019 relying on counting tax year 2018 as one of the three years of filing compliance, it would be prudent (perhaps absolutely necessary) to have filed a 2018 tax year return (if required) BEFORE an interview takes place.
The vast majority of applicants will have met the tax filing obligation for all previous years, of course, and thus have four years that count toward the required three years of compliance without considering 2018 at all. And even if they complied just three of those previous four years (remember, "NO" -- "NO" counts as a year of compliance, assuming the truthfulness of the "NO" not required to file response), they still meet the tax filing obligation requirement with NO regard for 2018.
Also note, for example, even if the applicant decides to nonetheless check "YES" required to file for 2018, early in 2019 and before filing a 2018 return, and checks "NO" did not file, NO further explanation is necessary as long as the applicant reports compliance for THREE of the FOUR previous years (three of the years 2014 to 2017).
