ההחלטה הזאת הוגנת.

Because the base noun is החלטה (decision), and the definite article ה־ (the) is added to the front.

So:

• החלטה = decision
• ההחלטה = the decision

The noun itself already begins with ה, so when you add another ה־, you get ההחלטה.

In Hebrew, demonstratives like this and that usually come after the noun.

So Hebrew says:

• ההחלטה הזאת = literally the decision this

but in natural English that becomes:

• this decision

This is a very common Hebrew pattern:

• הספר הזה = this book
• הילדה הזאת = this girl

Because החלטה is a feminine singular noun, and Hebrew words that describe or point to a noun usually have to agree with it in gender and number.

Here:

• החלטה = feminine singular
• זאת = feminine singular this
• הוגנת = feminine singular fair / just

So all the grammar matches.

One strong clue is the ending ־ה. Many Hebrew nouns ending in ־ה are feminine, and החלטה is one of them.

That said, the ending is only a clue, not an absolute rule. But in this sentence, you can clearly see it is feminine because the other words agree with it:

• זאת rather than זה
• הוגנת rather than הוגן

In present-tense Hebrew, the verb to be is usually not written or said in simple sentences like this.

So:

• ההחלטה הזאת הוגנת

literally looks like:

• the decision this fair

but it means:

• this decision is fair

This is normal Hebrew grammar. In past or future, Hebrew does use forms of to be, for example with היה.

Because הוגנת here is a predicate adjective: it tells you something about the subject.

So this sentence means:

• The/this decision is fair

In that kind of structure, the adjective does not take ה־.

Compare:

• ההחלטה הזאת הוגנת = This decision is fair
• ההחלטה ההוגנת = the fair decision

In the second example, ההוגנת is part of the noun phrase itself, so it does take ה־.

After a noun, Modern Hebrew normally uses the demonstrative forms:

• הזה = this (masculine)
• הזאת = this (feminine)
• האלה = these

So ההחלטה הזאת is the normal everyday way to say this decision.

Do not translate it word-for-word as the this decision. In this pattern, הזאת is just the normal form of this after the noun.

Yes. ההחלטה הזו is also very common in Modern Hebrew, especially in everyday speech.

Very roughly:

• ההחלטה הזאת = very standard, common
• ההחלטה הזו = also very common, often a bit more colloquial
• החלטה זו = more formal/written style

All of these can mean this decision, but the sentence you were given uses one of the most standard everyday patterns.

The masculine singular form is הוגן.

So:

• הוגן = masculine singular
• הוגנת = feminine singular

For example:

• החוק הזה הוגן = This law is fair
• ההחלטה הזאת הוגנת = This decision is fair

This is a good example of adjective agreement in Hebrew.