Learn Croatian: Fizički rad je zdrav, ali težak. - Physical work is healthy but hard.

Fizički (physical) is a relational adjective classifying the type of work, and these always take the definite -i ending in the masculine nominative. However, zdrav (healthy) and težak (hard) are used here as predicate adjectives (they come after the verb je to describe the subject). In Croatian, masculine adjectives used as predicates must take the indefinite (short) form, which drops the final -i.

I learned that težak means "heavy". Does it also mean "hard"?

Yes! While težak literally translates to "heavy" (like a heavy suitcase), it is universally used in Croatian to mean "hard" or "difficult" when talking about tasks, situations, or work. If you wanted to say "hard" in the sense of a solid object (like a rock), you would use the word tvrd.

Why is there an -a- in težak, but the feminine form is teška?

This is an example of the "fleeting a" (nepostojano a). Croatian avoids ending words with awkward consonant clusters like -šk. To make it easier to pronounce, an -a- is inserted before the final consonant in the masculine nominative singular. When you add an ending for other genders or cases (like the feminine -a to make teška), the cluster is no longer at the very end, so the extra -a- disappears.