Saints in the early Church needed frequent reminders (as do we all) that there's a difference between the Lord's way of teaching, and the way that often comes naturally. Disciples of Christ have received a divine mandate to preach His Gospel to all the world (see Matthew 28:19-20). However, as important as it is to be willing to boldly declare the word to others, it's just as important to know when to stay silent.
We are not without example from the master teacher Himself. There were numerous occasions when Jesus remained silent, at times even refusing to answer a direct question (for example, Matthew 26:63, St John 19:9, Luke 23:9). The pattern for these cases seems to be that the listener or questioner was not spiritually prepared to receive instruction. In addition to this possibility, we have our own spiritual preparedness to be concerned about. Hatred, pride, or anger can impede our own ability to teach. If at any time we feel that the Holy Spirit is not present to carry the truth into the hearts of of those with whom we are speaking, it is better to exercise restraint and forbear.
The statement that we "shall not teach" if we receive not the Spirit could be seen as both a commandment and a warning. We are commanded not to teach if we receive not the Spirit, and we are warned that if we attempt to do so anyway, we won't teach, because the one we are trying to teach won't learn anything. After all, no one can be forced to understand, much less accept truth. And attempting to force it can actually create a stumbling block, making it more difficult to reach someone in the future.
There may be times when we are speaking to one who is not receptive, and yet we feel prompted to continue teaching. It could be that they will remember the words taught to them at a later time when they are more open to it. There may be others present who need to hear it. Or it may simply be for our own benefit, to strengthen our testimonies and help plant the seeds of truth even deeper into our own hearts. If we suspect that this may be the case, we should examine our hearts and motives, and sense whether the Spirit is indeed present. When in doubt, we can be bold and speak up, provided we do so with humility and love. But when we can tell the Spirit is not present, we should save our teaching for a time when He is.