Learn Finnish: Talouskasvu hidastunee silti. - Economic growth is likely to slow down nevertheless.

The verb is hidastua (to slow down). Its strong vowel stem is hidastu-. To form the potential mood, we add the marker -ne- followed by the personal ending. Since the subject is talouskasvu (it, 3rd person singular), the vowel lengthens, resulting in the ending -e. This gives us hidastunee. Because the stem ends in a vowel, there is no consonant assimilation here.

Why does the English translation say "is likely to" when there is no separate word for "likely" in the Finnish sentence?

That is the core function of the potential mood! The -ne- marker bakes the meaning of "probably," "likely," or "might" directly into the verb itself. You do not need to add adverbs like ehkä (maybe) or todennäköisesti (probably), though Finns sometimes use them alongside the potential mood for extra emphasis.

Could I use the verb hidastaa instead of hidastua here?

No, because they have different grammatical roles. Hidastua is intransitive, meaning something slows down on its own (like economic growth). Hidastaa is transitive, meaning someone or something slows something else down. The -stu- suffix in hidastua is a very common way to form verbs that mean "becoming [adjective]" from words like hidas (slow).

Can I use kuitenkin instead of silti at the end of this sentence?