form ます and the form て


Can someone help me understand if the difference in use between the form ます
and the form て in this kind of sentence? 魚が泳いでいます。 (It's the same as when we
say in English that we're doing something?) 魚が泳ぎます。  
(It's like attesting a fact?)




First off, the "て form" in Japanese refers to just the form 泳いで, or the form
of the verb ending in て or で。When you write a verb like 泳いでいます, that's called
te-iru/te-imasu form or sometimes "progressive form," although that can be
misleading.

Your understanding is nearly correct. 魚が泳いでいます means "The fish is swimming."
As in, right now, a particular fish is swimming.

魚が泳ぎます is a little trickier. It doesn't mean "Fish swim" as a general property
of fish. That would be:

> 魚 **は** 泳ぎます。

The は particle is used for attesting a well-known fact.

魚が泳ぎます most likely means one of these two things, and you'd need context to
determine which one:

  1. The fish will swim.
  2. It's the answer to the question 何が泳ぎますか。What swims? Fish swim.

Finally, the te-imasu form can be used for a habitual action, although it
doesn't make sense in this case. But I could say 最近泳いでいます, meaning that I've
been going swimming lately.

