The short answer is: We don’t.
How do you know that “He goed” is not grammatically correct? At some point in their infancy, most children will say “He goed”. Then they stop saying it, long before a school teacher tells them that it is not good grammar. Good grammar, like vocabulary, is acquired before it is learned. Those who consistently use correct grammar, whether in their first language or their target language, have been immersed in a social network made of those who habitually use correct language, or they have acquired correct grammatical structures through reading. Like vocabulary, grammar is acquired through Comprehensible Input, either oral or written. Later, much later, we may find it amusing to study grammar systems and use expressions like gerunds and imperfect conditional subjunctives, but the vast majority of human beings can survive without ever knowing what a subjunctive is or where it lives.
And this is why reading is so important to acquisition. Written texts are usually grammatically correct, so they give our students high quality input. As a very young student I quickly realized that whether or not I understood the grammatical rules, I could get a perfect score on any grammar test by selecting whichever answer “sounded right.” I was a reader. What sounded “right” was what sounded like something I had read. Readers acquire good grammar, whether or not they memorize the rules found in textbooks, just as children acquire the local accent without being taught it in Speech Class.
One of the main strengths of Comprehensible Input strategies is that vocabulary is always presented in context, in the context of a story or a message, and never in lists. Thus vocabulary is always approached in its natural grammatical habitat and the correct “conjugation” is acquired naturally. Many many years ago, when TPRS teachers were a small minority and realized that their students would be moving on to more traditional teachers who expected them to fill in conjugation charts, they discovered that they could simply show their students what a conjugation chart was at the end of the year. Their students had already acquired the correct verb forms without ever having studied or seen such charts. It took only a few days to show them how to put what they had Acquired into the charts that other students had spent all year trying to memorize.
A popular way to prepare students for more extensive grammar study with other teachers is “pop-up grammar.” The teacher will point out a grammatical structure and demonstrate how it can modify the meaning of a phrase. The key is always meaning. “He goes to work in the morning” does not have the same meaning as “He went to work Thursday morning.” An explanation of how a grammatical choice can change the meaning should last less than a minute.
My own personal exception to the rule of minimal grammar is if a student asks me a question. Personally, I find grammar fascinating and occasionally you will have a student that shares the same interest. I always answer such questions from students as thoroughly as they can bear it. My teaching philosophy was heavily influenced by a study showing that students always remember the answers to their own questions. So I never waste time answering questions that no one has asked, but I always give them the answers, and then some, to the questions that they themselves ask.
*Because the role of correct, high quality Comprehensible Input is so important in the Acquisition of language, whether spoken or written, CI teachers frown on the use of forced output, often called “Communicative Activities”, which expose students to vast amounts of incorrect, low quality output from other students.