In this post, I explain the case marking system with special reference to diptotes or (الممنوع من الصرف). Diptotes refers to a category of nouns and adjectives that do not take tanween when they are indefinite and have special markings for cases. It should be remembered that when these words are definite, they take regular case markings.
For example, the word (مدارس) is a diptote, and when it is indefinite it marked with (ـُ) for the nominative case and with (ـَ) for both accusative and genitive cases.

هذه مدارسُ كبيرةٌ.

These are large schools.

زرت مدارسَ كبيرةً.

I visited large schools.

درست في مدارسَ كبيرةٍ.

I studied at large schools.

Notice how the cases are marked differently on the nouns that are diptotes and the adjectives that take regular case markings!

Broken plurals that contain three or more syllables are diptotes, for example broken plurals of the patterns مفاعل  فواعيل – فعاليل – –  مفاعيل –  فواعل, for example (مساجد) ‘mosques’, (تلاميذ) ‘pupils’, (قواميس) ‘dictionaries’, (جماهير) ‘audiences’, (تماثيل) ‘statues’, etc.

Now, try to decide which words in the sentences below are diptotes and try to vowel them.

استخدمنا الكثير من المراجع وقواميس عديدة لكتابة البحث.

هؤلاء تلاميذ جدد في المدرسة وقد جاءوا من مدارس مختلفة.

فقدت مفاتيح بيتي ولكنني وجدتها في قسم الشرطة.

قامت الحكومة ببناء مصانع جديدة ومعاهد فنية في المدينة.

Scroll down for the answers, and the translation of sentences:

استخدمنا الكثير من المراجعِ وقواميسَ عديدة لكتابة البحث.

We used many references and various dictionaries to write the research paper.

هؤلاء تلاميذُ جدد في المدرسة وقد جاءوا من مدارس مختلفة.

These are new pupils at the school, and they came from different schools.

فقدت مفاتيحَ بيتي ولكنني وجدتها في قسم الشرطة.

I lost my house keys but I found them in the police station.

قامت الحكومة ببناء مصانعَ جديدة ومعاهدَ فنية في المدينة.

The government built new factories and vocational institutions in the city.

There are other categories of words that are diptotes. They will be explained in the next post or two.