هذه الوثيقة مهمة.

• هذه = this (feminine singular)
• الوثيقة = the document
• مهمة = important

So the structure is literally:

• this
  • the document
    • important

Natural English: This document is important.

In Arabic, this is a nominal sentence, so there is no separate word for is in the present tense.

Because الوثيقة is a feminine singular noun.

In Arabic, demonstratives must agree with the noun they refer to:

• هذا = this for masculine singular
• هذه = this for feminine singular

Since وثيقة is feminine, Arabic uses هذه.

A very common clue is the ending ة (called tāʾ marbūṭa).

• وثيقة ends in ة
• nouns with this ending are very often feminine

So وثيقة is grammatically feminine, and that affects the form of:

• the demonstrative: هذه
• the adjective: مهمة

Keep in mind that grammatical gender in Arabic is a language feature, not necessarily something logical or natural.

Because adjectives in Arabic must agree with the noun they describe.

Since الوثيقة is:

• feminine
• singular
• definite

the adjective must match it in gender and number, so we get:

• مهمة = feminine singular important

If the noun were masculine, you would use مهم instead.

Example:

• هذا كتاب مهم = This is an important book / This book is important

In Arabic, the verb to be is usually not expressed in the present tense.

So:

• هذه الوثيقة مهمة literally looks like This document important
• but it means This document is important

This is completely normal in Arabic.

If you want past or future, Arabic does use verbs:

• كانت الوثيقة مهمة = The document was important
• ستكون الوثيقة مهمة = The document will be important

That is the normal pattern in Arabic.

English:

• an important document

Arabic:

• وثيقة مهمة
• literally: document important

So in this sentence:

• الوثيقة = the noun
• مهمة = the adjective describing it

This noun-then-adjective order is one of the basic patterns learners need to get used to in Arabic.

This is because the whole sentence is not saying the important document. It is saying the document is important.

There are two different patterns here:

1. Predicate sentence

   • هذه الوثيقة مهمة
   • This document is important
   • Here مهمة is the predicate, so it is usually indefinite

2. Noun phrase

   • هذه الوثيقة المهمة
   • this important document
   • Here المهمة is an adjective directly inside the noun phrase, so it matches definiteness too

So:

• الوثيقة مهمة = the document is important
• الوثيقة المهمة = the important document

That difference is very important in Arabic.

It functions together as this document.

In Arabic, the demonstrative often comes before a definite noun:

• هذه الوثيقة = this document

Notice that the noun is usually definite:

• هذه الوثيقة not usually
• هذه وثيقة in the same basic sense

So you can think of هذه + definite noun as a very common pattern for this + noun.

In fully vocalized formal Arabic, it would commonly be written as:

• هٰذِهِ الْوَثِيقَةُ مُهِمَّةٌ

The endings show grammar:

• الوثيقةُ has -u because it is the subject/topic of the nominal sentence
• مهمةٌ has -un because it is an indefinite predicate

But in normal modern writing, these short-vowel endings are usually omitted, so you usually see:

• هذه الوثيقة مهمة

Both represent the same sentence.

The final ة (tāʾ marbūṭa) is usually pronounced as -a or -ah when you pause at the end of the word.

So:

• وثيقة sounds roughly like wathīqa
• مهمة sounds roughly like muhimma

But when grammar endings are pronounced in fully vocalized formal Arabic, the ة behaves like a t before the ending:

• الوثيقةُ = al-wathīqatu
• مهمةٌ = muhimmatun

This is a very common feature of Arabic pronunciation and grammar.

In هذه, the final ه is part of the spelling of the word, but the word is pronounced hādhihi in careful MSA pronunciation.

So the ending is not just a simple English-style h sound added at the end. The whole word is an established demonstrative form:

• هذه = hādhihi

In slower or more careful reading, learners often hear:

• hā-dhi-hi

In actual speech, pronunciation can vary somewhat depending on style and dialect influence, but for MSA, hādhihi is the safe form to learn.

Yes, if the context already makes clear what this refers to.

• هذه مهمة = This is important or This one is important

But if you want to say specifically This document is important, then including الوثيقة is clearer:

• هذه الوثيقة مهمة

So both are possible, but they do not always mean exactly the same thing in context.

You would change both the demonstrative and the adjective.

For example, with كتاب (book), which is masculine:

• هذا الكتاب مهم
• This book is important

Compare:

• هذه الوثيقة مهمة = feminine
• هذا الكتاب مهم = masculine

So the pattern changes like this:

• هذه → هذا
• مهمة → مهم

A natural MSA sentence would be:

• هذه الوثائق مهمة

Here:

• هذه can also be used with non-human plurals
• الوثائق = the documents
• مهمة stays feminine singular because non-human plural nouns usually take feminine singular agreement in standard Arabic

That is a point that often surprises English speakers.

So:

• هذه الوثيقة مهمة = This document is important
• هذه الوثائق مهمة = These documents are important

In MSA, a common way is:

• هذه الوثيقة ليست مهمة

Here ليست is the feminine singular form of ليس, which is used for negation in nominal sentences.

So:

• هذه الوثيقة مهمة = This document is important
• هذه الوثيقة ليست مهمة = This document is not important

Because الوثيقة is feminine singular, Arabic uses ليست.