Help with correct translation
&quot;I&quot; (我) is the subject. It always go first
[I am playing tennis] is the main clause. You translated it as [我打网球], but it should be [我正在打网球] (you wrote &quot;playing&quot;, not &quot;play&quot;)
[in a red jacket] is the relative phrase. (giving additional information on how you are playing tennis) You translated it as [用红色的外套] is incorrect.

[用红色的外套] should be [穿着一件红色外套]

的 in 红色的 can be omitted

&quot;a red jacket&quot; is &quot;一件红色的外套&quot;


Put it together we get:

[我正在打网球]  [穿着一件红色外套]
[main clause] [relative phrase]

Unlike in English, relative phrase usually precede main clause in Chinese grammar.
The correct translation of &quot;I am playing tennis in a red jacket.&quot; should be:

&quot; 我正(穿着一件红色外套)打网球&quot;
在 in 正在 can be omitted

It would be simpler if you wrote in present tense
&quot;I play tennis in a red jacket&quot; --&gt;&quot; 我 (穿着一件红色外套)打网球&quot;
Edit:
Vegar Reitan wrote:

How do I know where to put the relative sentence?

You have to decide which verb phrase in a sentence is the main phrase- If you wrote:&quot; I eat apple at home&quot;, You can see [eat apple] is the main action, [at home] is added info
The English sentence [ I eat apple (at home) ] would become [ I (at home) eat apple] [我(在家)吃苹果] in Chinese grammar
Vegar Reitan also wrote:

And how do you know that &quot;的 in 红色的 can be omitted&quot;?

One of the functions of '的' is being an adjectival particle that mark a phrase or clause before as an adjectival phrase or clause

The adjectival particle '的' in '红色的' can be omitted when it is followed by an object, because 红 or 红色 itself is an adjective, it doesn't require the adjectival particle '的' to function as an adjective.

Example: (红色的)外套 can be omitted to (红色)外套 or (红)外套

The adjectival particle '的' in '红色的' cannot be omitted when it is preceded by an object

Example: 这外套是(红色的) cannot be reducted to 这外套是(红色) or  这外套是(红)--  That would mean &quot;This Jacket is red(n)&quot;