Learn Farsi: این افکار بد پروژه را خراب می‌کنند.

The singular is fekr (thought). afkâr is an Arabic 'broken plural'. Instead of adding a standard Persian suffix like -hâ, the word's internal vowels change and extra letters are added to form the plural. You will see this specific pattern often (like shakhs becoming ashkhâs). You don't need to learn the rules to create them; just memorize the common ones as vocabulary.

Since the translation says 'these bad thoughts', why do we use the singular in instead of the plural inhâ?

In Farsi, when demonstratives like in (this) or ân (that) are placed directly before a noun, they act as adjectives and never take a plural suffix. Because the noun afkâr is already plural, in automatically translates to 'these'. You would only use inhâ if it were standing alone as a pronoun (e.g., 'These ruin the project').

Why does the verb kharâb mikonand have a plural ending if 'thoughts' are inanimate objects?

You might remember from earlier lessons that inanimate plural subjects in Farsi usually take a singular verb. However, using a plural verb with inanimate abstract nouns (like thoughts) is also grammatically correct and very common in modern Farsi, especially when those things are actively 'doing' something, like ruining a project. Both kharâb mikonad (singular) and kharâb mikonand (plural) are acceptable here.

How are the words afkâr and bad connected when speaking?