אני חושב שהסיכום שלך די ברור, אבל הייתי כותב אותו קצר יותר.

Hebrew present-tense verb forms agree with the speaker’s gender.

• A male speaker says אני חושב = I think
• A female speaker says אני חושבת

The same thing happens later in the sentence:

• male: הייתי כותב
• female: הייתי כותבת

So the sentence as written sounds like it is being said by a male speaker.

The prefix ש־ means that.

So:

• אני חושב = I think
• שהסיכום שלך די ברור = that your summary is quite clear

This ש־ is extremely common in Hebrew and is attached directly to the next word.

In Hebrew, possession with של usually comes after the noun:

• הסיכום שלך = your summary
• literally, something like the summary of yours

That is the normal word order in Modern Hebrew.

Because the phrase refers to a specific summary: your summary.

In Hebrew, possessed nouns are often definite, so you usually get:

• הספר שלי = my book
• הסיכום שלך = your summary

Without ה־, סיכום שלך would usually sound less natural here.

די means quite, pretty, or fairly.

So:

• די ברור = quite clear / pretty clear

It slightly softens the statement. It is less strong than saying the summary is completely or very clear.

Because ברור agrees with הסיכום.

The noun סיכום is:

• masculine
• singular

So the adjective must also be masculine singular:

• סיכום ברור = a clear summary

If the noun were feminine, you would use ברורה.

This is a very common Hebrew way to express would.

Literally, הייתי כותב is something like I was writing, but in real usage it often means I would write in a hypothetical or softened sense.

So here:

• אבל הייתי כותב אותו קצר יותר = but I would write it shorter

This sounds less direct and less blunt than a straight command or a stronger statement.

אותו here means it.

It refers back to הסיכום.

Because סיכום is masculine singular, the pronoun is also masculine singular:

• masculine singular: אותו = it / him
• feminine singular: אותה = it / her

So כותב אותו means write it.

קצר יותר means shorter.

Hebrew often forms comparisons with יותר:

• קצר = short
• קצר יותר = shorter / more short

So:

• כותב אותו קצר יותר = write it shorter

Yes, both יותר קצר and קצר יותר are used.

In this sentence, קצר יותר sounds very natural.
You may also hear:

• הייתי כותב אותו יותר קצר

Both are understandable and common. The version in your sentence is a nice, standard way to say it.

You can say that, and it is very natural.

• הייתי מקצר אותו = I would shorten it

The sentence you have, הייתי כותב אותו קצר יותר, puts the focus on how I would write it.
הייתי מקצר אותו focuses more directly on making it shorter.

So the meaning is close, but the nuance is slightly different.

Yes, fairly polite.

Several things soften the criticism:

• אני חושב = I think
• די ברור = quite clear
• הייתי כותב = I would write

So instead of sounding harsh, it sounds like constructive feedback:

• first a positive comment
• then a mild suggestion for improvement

That is a very natural way to give feedback in Hebrew.