Learn Finnish: Älä koskaan aliarvioi oireita, jos olet yliherkkä lääkkeelle. - Never underestimate the symptoms if you are hypersensitive to the medicine.

The prefix ali- means 'under' or 'sub-'. Combined with the verb arvioida (to estimate or evaluate), it becomes aliarvioida (to underestimate). In this sentence, it appears as aliarvioi, which is the connegative form required by the negative imperative word älä.

What does the prefix yli- add to the word yliherkkä?

The prefix yli- means 'over' or 'excessive'. When attached to the adjective herkkä (sensitive), it forms yliherkkä (hypersensitive or allergic). It functions very much like the English prefixes 'over-' or 'hyper-' to show that a quality is present to an extreme degree.

Why is the object oireita in the partitive case?

Because the sentence is a negative command (älä koskaan aliarvioi). In Finnish grammar, the object of a negative sentence almost always takes the partitive case. Oireita is the partitive plural of oire (symptom).

Why does lääke take the allative ending -lle in this sentence?