Learn Turkish: Bu ilacın uyarıcı olduğunu duydum. - I heard that this drug is a stimulant.

It is built from the verb root uyar- (from uyarmak, meaning to stimulate or warn) and the agentive suffix -ıcı. This suffix turns the verb into a noun or adjective describing something that performs the action, giving us 'stimulant' in this context.

I usually see uyarmak translated as 'to warn.' Why does it mean 'stimulant' here?

Uyarmak has two main meanings: 'to warn' and 'to stimulate' or 'to arouse' (often in a medical, biological, or scientific sense). Because the context here is about a drug (ilaç), the derivative noun uyarıcı naturally translates to 'stimulant' rather than something that issues a warning.

Why does ilaç take the genitive suffix to become ilacın?

It is the subject of the embedded clause 'that this drug is a stimulant'. In Turkish, when you form a subordinate 'that' clause using a participle like -dık (which is inside olduğunu), the subject of that specific clause must take the genitive case.

What are all the suffixes attached to olduğunu?

The root is ol- (to be). The -duğ- is the mutated form of the object participle suffix -dık. The first -u is the 3rd person possessive suffix linking back to ilacın (literally 'its being'). Then there is a buffer letter -n-, followed by the accusative suffix -u because the entire clause is the direct object of the main verb duydum (I heard).