“~nnda” is the casual version of the expression “noda” (the polite expression is “no desu”). It is often used at the end of sentence, as in, “sou nannda” (Is that right?) or “shikata nai nnda ne” (It can’t be helped). It differs from “sou da” or “shikata nai ne” because the reason or cause of what has just been said is stated just before or is implied by the situation. For instance, when you are invited to participate in a game and decline, you might say, “ashita, tesuto nannda” (I have a test tomorrow). When looking out a window, and you discover that the road is wet, or that someone is holding an umbrella, on reaching the conclusion that it is raining, you might say, “a, ame ga futteru nnda ne” (Oh, it’s raining). “kyou, tesuto da ne” and “a, ame ga futteru” are simply facts for which it’s difficult to divine the reason why they were stated. In addition, when you use “~nnda,” the person you are talking to gets a sense that you intend to explain or are informing them of a certain situation. However, you should be aware that it will be interpreted as an excuse if you use it too much. An example of bad usage: “okureta noha betsu no youji ga atta nnda. sore wo kyou owaraseru youni iwareta nnda. watashi ha, isoida nnda yo. demo, jikann ga kakatta nnda.” (The reason I was late is that I had other stuff to do. I was told I had to finish it today. I hurried to get here, but it took too much time.) An example of correct usage: “okureta noha, betsu no youji ga atte ne. sore wo kyou owaraseru youni iwareta nnda. You will make a good impression by adding these kind of apologies. It is also used when bringing matters to a conclusion, as in, “kini suru na. kore de ii nnda. (Don’t worry, it’s OK.) And it is also used to show decisiveness, as in “ore ga yaru nnda.” (I’ll definitely do it.) isoida kedo, jikann ga kakatte, gomenn ne.”