atarasii mausu ha tiisakute, te ni aimasu.

は marks マウス as the topic of the sentence: as for the new mouse...

So the sentence structure is basically:

• 新しいマウスは = as for the new mouse
• 小さくて、手に合います = it is small and fits the hand

A natural literal breakdown is:

• 新しいマウスは = the new mouse
• 小さくて = being small / and small
• 手に合います = fits the hand

Japanese often uses は when introducing what you are talking about, rather than making it a strict grammatical subject the way English does.

Because 新しい is directly modifying the noun マウス.

In Japanese, when an い-adjective comes before a noun, it stays in its plain form:

• 新しいマウス = a new mouse
• 小さい部屋 = a small room
• 高い店 = an expensive shop

You do not use です before a noun like that.

Politeness is shown at the end of the sentence here with 合います. It is very normal in Japanese for the sentence to be polite overall even though words inside the sentence are plain forms.

小さくて is the て-form of the い-adjective 小さい.

For い-adjectives, the て-form is made by changing -い to -くて:

• 小さい → 小さくて
• 新しい → 新しくて
• 高い → 高くて

The て-form connects one idea to another. Here it links:

• small
• fits the hand

So 小さくて、手に合います means something like:

• It is small, and it fits the hand
• Because it is small, it fits the hand

Depending on context, the て-form can sound like simple connection or a loose reason.

Because 小さくて is not the end of the sentence. It is just connecting to the next part.

In Japanese, only the final predicate usually carries the main politeness marking. So this is natural:

• 新しいマウスは小さくて、手に合います。

Not:

• 新しいマウスは小さくてです、手に合います。 ← incorrect

If 小さい were the end of the sentence, then you could say:

• 新しいマウスは小さいです。 = The new mouse is small.

But once it becomes a connector, it changes to 小さくて.

手に合います means it fits the hand or it suits the hand.

Here, 合う does not mean physically combine two things like puzzle pieces. It often means:

• to suit
• to match
• to fit

So with a mouse, 手に合う suggests:

• it feels right in your hand
• its size/shape suits your hand
• it is comfortable to hold

It is a very natural way to talk about tools, clothes, glasses, shoes, furniture, and so on.

With 合う, the particle に often marks the thing that something suits or fits.

Examples:

• この靴は足に合う。 = These shoes fit my feet.
• このメガネは顔に合う。 = These glasses suit my face.
• このマウスは手に合う。 = This mouse fits the hand.

So 手に is not to the hand in a simple direction sense here. It is part of the pattern N に 合う = to suit / fit N.

This is something best learned as a set expression.

Both can be possible in different situations, but 合います is the simpler, more neutral choice.

• 合います = fits / suits
• 合っています = is fitting / is suited / is in a state of fitting

For a general statement about the mouse, 合います is very natural:

• This new mouse is small and fits the hand.

If someone wants to emphasize the current state or result, 合っています could appear in some contexts, but it is not necessary here.

For learners, it is safest to understand 手に合います as the standard way to say it fits the hand.

It can suggest either, depending on context.

The て-form often has a broad linking function. In this sentence, English might translate it as:

• It is small, and it fits the hand.
• It is small, so it fits the hand.

Both are reasonable.

Because the two ideas are closely related, many learners feel a slight reason/cause nuance here: its small size is one reason it fits the hand well.

Japanese often leaves that relationship a little less explicit than English.

Japanese very often leaves out pronouns when the meaning is clear from context.

In English, you would normally say:

• The new mouse is small, and it fits the hand.

In Japanese, once 新しいマウス has been introduced as the topic, there is no need to repeat it:

• 新しいマウスは小さくて、手に合います。

This omission is one of the most common differences between English and Japanese sentence structure.

Yes. マウス is a loanword from English mouse, and in this sentence it means a computer mouse.

Katakana is commonly used for foreign loanwords:

• マウス = mouse
• コンピューター = computer
• テーブル = table

The pronunciation is adapted to Japanese sound patterns, so マウス is pronounced more like ma-u-su.

Yes, it is natural and easy to understand.

It sounds like something you might say when describing a computer mouse:

• The new mouse is small and fits the hand well.

If you wanted to sound a little more explicit in English, the nuance could be:

• The new mouse is small and comfortable to hold.

But the Japanese sentence itself is completely normal as written.