Like in English we have adjectives in Gujarati , to describe more about noun/pronoun. Lets see rules for usage of adjectives.

Rule 1) In Gujarati usage of adjective can change according to the gender and multiplicity of noun. Generally those masculine adjective which ends with vowel ઓ (o) change as per gender and multiplicity.

So these forms follow “ઓ-ઈ-ઉં-આ-ઈ-આં” pattern discussed in lesson
ઓ-ઈ-ઉં-આ-ઈ-આં (o-I-uM-A-I-AM ) યો-ઈ-યું-યા-ઈ-યાં ( yo-I-yuM-yA-I-yAM ) rule

e.g. Black -> કાળો (kALo )
Black horse -> કાળો ઘોડો (kALo ghoDo )
Black chair -> કાળી ખુરશી (kALI khurashI )
Black plank -> કાળું પાટિયું (kALuM pATiyuM )

In Gujarati Horse i.e.ઘોડો is masculine , chair i.e. ખુરશી is feminine and plank i.e. પાટિયું is neuter. So the adjective was changed to કાળો, કાળી , કાળું respectively.

For plurals they will be
Black horses -> કાળા ઘોડા (kALA ghoDA )
Black chairs -> કાળી ખુરશીઓ (kALI khurashIo )
Black planks -> કાળાં પાટિયાં (kALAM pATiyAM )

So in plural forms changed to કાળા ,કાળી ,કાળાં

These forms are created same as we had formed forms of verbs in past tense, હતો-હતી-હતું-હતા-હતી-હતાં (hato-hatI-hatuM-hatA-hatI-hatAM).

Similarly in “going to phrase” lesson we had seen form
બોલવાનો-બોલવાની-બોલવાનું-બોલવાના-બોલવાની-બોલવાનાં (bolavAno-bolavAnI-bolavAnu-bolavAnAM-bolavAnI-bolavAnAM) etc.
So nothing new in this concept.

Rule 2 ) Other adjectives generally remain same in all genders.

e.g. Red -> લાલ(lAl)
Red horse -> લાલ ઘોડો (lAl ghoDo )
Red chair -> લાલ ખુરશી (lAl khurashI )
Red plank -> લાલ પાટિયું (lAl pATiyuM )

In Gujarati Horse i.e.ઘોડો is masculine , chair i.e. ખુરશી is feminine and plank i.e. પાટિયું is neuter. So the adjective was unchanged.

For plurals they will be
Red horses -> લાલ ઘોડા (lAl ghoDA )
Red chairs -> લાલ ખુરશીઓ (lAl khurashIo )
Red planks -> લાલ પાટિયાં (lAl pATiyAM ).

Even in plurals adjective was unchanged.

Rule 3) When adjective ending with vowel ઓ (o) is used along with preposition, its form changes similar as if it was noun ending with ઓ (o)
Refer lesson (https://learnmarathiwithkaushik.com/courses/prepositions-with-nouns-in-gujarati/) for more details

e.g.
Good boy -> સારો છોકરો (sAro Chokaro)
With good boy -> સારા છોકરા સાથે (sArA ChokarA sAthe)

When preposition “With” is used with noun, we changed from “સારો”(sAro) to સારા(sArA).

Left pocket -> ડાબું ખીસું (DAbuM khIsuM)
In left pocket -> ડાબા ખીસામાં (DAbA khIsAmAM)
When preposition “in” is used with noun, we changed from “ડાબું”(DAbuM ) to ડાબા (DAbA ).

Good girl -> સારી છોકરી (sArI ChokarI)
With Good girl -> સારી છોકરી સાથે (sArI ChokarI sAthe)
As per rules, form of feminine word does not change so we used સારી(sArI) in both cases.

Rule 4) Other adjectives will remain same even with preposition

e.g.
Beautiful picture -> સુંદર ચિત્ર (suMdar chitr)
About Beautiful picture -> સુંદર ચિત્ર વિષે (suMdar chitr viShe)

Happy Family -> આનંદી કુટુંબ ( AnaMdI kuTuMb )
For Happy Family -> આનંદી કુટુંબ માટે ( AnaMdI kuTuMb mATe)

Rule 5) Adjectives from verb

For adjectives made from verb refer lesson
https://learnmarathiwithkaushik.com/courses/adjectives-made-from-verbs-in-gujarati/