The relative pronouns (الأسماء الموصولة) are used after definite nouns to provide additional information about them, and they are followed by complete sentences, e.g.

أحب الكتاب الذي أدرسه.

“I like the book which I study.”

أحب الكتابين اللذين أدرسهما.

“I like the books which I study.” (dual)

أحب الكتب التي أدرسها.

“I like the books which I study.” (plural)

Dual relative pronouns (اللذان/اللذين) masculine and (اللتان/اللتين) feminine can be tricky to use because they are often associated with certain case marks, i.e. in the nominative case, we should use اللذان and اللتان. In the accusative and genitive case, we should use اللذين and اللتين. Generally speaking, we use اللذان and اللتان after nouns that serve as subject of verbal sentences, or that serve as subject or predicate of nominal sentences; and we use اللذين and اللتين after nouns that serve as object of verbal sentences, or that follow a preposition or act as second noun of idafa construction.

Now, try to use this information to complete the sentences below with the appropriate form of dual relative pronouns, i.e. اللذان, اللتان, اللذين, and اللتين.

1-     هذان هما الطالبان ———- يدرسان في جامعتي.

2-      تحدثت مع المهندستين ———— تعملان في المشروع الجديد.

3-      قرأت كتب الكاتبين ———— عاشا في القرن الثامن عشر.

4-      زرت المدينتين —————- تقعان على شاطئ البحر.

5-      الأستاذان ————— يدرسان اللغة العربية في جامعتي ممتازان.

Tip: You need to decide on the gender of the noun first to choose between (اللذان/اللذين) masculine and (اللتان/اللتين) feminine. Then, you need to decide on the function of the noun in the sentence to choose between the ان forms ending in ين for nominative or those ending in  for accusative and genitive.
I will post the answers of this exercise with more exercises on other forms of relative pronouns in my next post!