1. Sure, you can start by learning the dictionary forms for conjugation, and the Japanese people will think you are cute for trying. But you still look like you’re stumbling around the language. Why choose that on purpose?

2. If you start with the ~masu forms, you automatically look like you’ve actually studied the language at least a bit, and even the stumbling looks more polished. Plus, as you work on learning other forms it gives you a default to fall back on. That way your default level is tuned to “polite” and sure, it’s too formal for some situations, but having been here a few months (with years to go) I’m already seeing the benefits of knowing ~masu form over the more informal ones.

3. Americans in particular are used to a much less formal and polite method of speaking. By default, our language is very wordy, very familiar, and that’s fine. But the Japanese people have a different mentality to start with. If you come at it from an American perspective then sure, starting informal and working up is fine, and it feels weird to be all polite. But from a Japanese perspective, you’re showing them respect, and they’ll respect that and show you the same in return. It establishes a good framework from the start. If you start out all “yo, dog” to a Japanese person, it looks like you haven’t bothered to care about their society and culture.

Just my two cents. Again, years late, but I hope this blog is updated again since I really enjoy reading the posts!

For me, I use dictionary form with friends and co-workers I’m closer with, and masu and desu form for those I don’t know so well. The guys in my team at work are generally pretty laid-back, and use casual form between themselves all the time. Likewise, you’ll hear guys use boku and (less frequently) ore rather than watashi.

After a few months living here and having only studied polite forms at that point, some Japanese friends of mine asked me why I was always so overly formal using desu and masu all the time. Using casual forms are in a way an expression of how you view your relationship with them, so you can seem distant if you refrain from using them.

All that said, the expectations for foreigners’ Japanese is generally not too high. As long as you can get your point across, you’re doing very well indeed.

When Japanese toddlers learn their native tongue, those are the forms they start with. I know my daughter uses them heavily at age two and a half. Linguistically, they’re more helpful to the learner as a foundation for future learning, in my opinion.

The rudeness of the dictionary forms isn’t something to be concerned about at the stage where the speaker can’t string together meaningful, grammatical sentences of any length anyway. Japanese listeners aren’t going to be offended by the beginner’s dictionary-form mumblings any more than they’re offended by my daughter for her ungrammatical ways.

That said, my first teachers and textbooks started me off on ですます調 because that’s the way of the world in formal JFL education. I was lucky to be in Japan, though, so I had plenty of extracurricular learning time to pick up the language in its other forms at the same time.

I have been in numerous situations where the use of the slightly more polite desu and masu have earned respect, as people recognize that I have a strong command of the language. This can also be invaluable in getting barriers to break down. Especially in business or in dealings with people in banks, shops, and a variety of other places that you will need to use Japanese on a daily basis. Using the basic form in these kind of situations can be considered quite rude.

There is no real reason for you to use desu and masu forms, and your japanese teacher simply told you there was because of the long held belief by japanese teachers that foreigners ought to use polite form all the time. I only felt like I was even getting to basic level of fluency once I discovered the basic form and all the wonderful things you could do with it!

From a language acquisition view I think it is better to learn the basic form and then move on to changing the verb endings to all the various forms than to learn one specific verb ending and have to backtrack to the simplified form.

Of course, there are situations even in a gaijin life where keigo is appropriate, but 95% of the time standard language will sound a lot more natural.