2 Corinthians 5:17 (NKJV)
Understanding grammar is important, especially when learning a language, and one of the things you learn about in grammar is the various tenses of past, present, and future. Past tense is of course something that already happened, present tense is what is happening now, and future tense deals with what will happen sometime later. How these things are expressed in each language can vary, but to be able to communicate correctly you have to understand the proper use of tenses.
Past tense, present tense, and future tense are not only important in a language, but it is an important concept in Christianity as well. If we do not have a proper understanding of what God has done for us through Jesus, which would be past tense, then we will not have a proper understanding of our present situation, and of what is coming in the future. However, if you talk to some Christians, they put some things that are past or present tense into a future tense.
In 2 Corinthians 5:17, it talks about someone who is in Christ, present tense, and that the old things have passed away and all things have become new. We are now a new creation in Christ, this is not something that we are looking forward to in the future. So often, Christians talk and act like all of the things provided in the past through Jesus are waiting for us some day in the future, but those things belong to us today! We need to understand everything that God has done for us through Jesus and recognize who we are in Christ as a present tense reality.