هذه علامة واضحة.

Because علامة is a feminine singular noun, and the demonstrative has to match it.

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

So with علامة, you say:

• هذه علامة...

not

• هذا علامة...

A very common clue is the ending ـة (called taaʾ marbuuṭa). Many nouns ending in ـة are feminine.

So:

• علامة ends in ـة
• therefore it is treated as feminine

That is why both the demonstrative and the adjective are feminine too.

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

They usually match in:

• gender
• number
• definiteness
• and, in fully vocalized Arabic, case

Since علامة is:

• feminine
• singular
• indefinite

the adjective must match it:

• علامة واضحة

If the noun were masculine, you would use the masculine adjective form instead.

In Arabic, descriptive adjectives normally come after the noun, not before it as in English.

So Arabic says:

• علامة واضحة

literally: sign clear

This is the normal pattern:

• noun first
• adjective second

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

So Arabic says:

• هذه علامة واضحة

without a separate word for is.

This is very normal in Modern Standard Arabic.
If you wanted the past, then a verb would appear, for example with كان.

Yes. In Arabic, this is a normal nominal sentence.

You can think of it as having:

• هذه = the topic / subject-like element
• علامة واضحة = the information being said about it

So it is fully grammatical and complete even though there is no present-tense is.

Because they are indefinite here.

• علامة واضحة = a clear sign
• العلامة الواضحة = the clear sign

Arabic marks definiteness very clearly.
If the noun is definite with الـ, the adjective also becomes definite:

• العلامة الواضحة

If the noun is indefinite, the adjective stays indefinite too:

• علامة واضحة

That ending is called taaʾ marbuuṭa (ة). It often marks feminine words.

In this sentence:

• علامة is a feminine noun
• واضحة is a feminine adjective agreeing with it

In pronunciation, this ending often sounds like:

• -a / -ah when pausing
• but as -t when followed by case endings or in certain grammatical contexts

So in careful full pronunciation, you may hear something like:

• ʿalāmatun
• wāḍiḥatun

But in normal pause:

• ʿalāmah
• wāḍiḥah

In fully vocalized formal Arabic, it is typically:

• هذهِ علامةٌ واضحةٌ

Both علامةٌ and واضحةٌ are in the nominative here.

A learner should also know that in everyday reading and speech, these final case endings are often not pronounced clearly, especially when pausing. So you will very often hear something closer to:

• hādhihi ʿalāmah wāḍiḥah

rather than the fully inflected:

• hādhihi ʿalāmatun wāḍiḥatun

In Modern Standard Arabic, هذه is usually pronounced:

• hādhihi

A few notes:

• هـا gives hā
• ذ is like the th in this
• the final ه here is part of the word’s standard pronunciation

So the whole sentence in careful pronunciation is approximately:

• hādhihi ʿalāmatun wāḍiḥatun

Yes, but it means something different.

• هذه علامة واضحة = This is a clear sign
• هذه العلامة الواضحة = this clear sign

The first is a full sentence.
The second is usually a noun phrase, not the same kind of statement.

So adding الـ changes both the grammar and the meaning.

That is very common in Arabic when the second part is the predicate of a sentence.

A structure like:

• هذا كتاب
• هذه علامة واضحة

is perfectly normal, where the thing after the demonstrative is indefinite.

This often corresponds to English sentences like:

• This is a book
• This is a clear sign

So the indefiniteness is not a mistake; it is exactly what Arabic often does in this kind of sentence.

For learners, the most useful analysis is usually as a full sentence:

• هذه = this
• علامة واضحة = a clear sign

So: This is a clear sign.

Arabic grammar can describe this in formal ways, but at the learning stage, it is enough to recognize that:

1. هذه points to something feminine singular
2. the following words describe what it is
3. the present-tense is is omitted

That is the key pattern to remember.