• Basic forms
• 苦しむ — to suffer; to struggle; to be in pain / base form
• 苦しみます — (I) suffer; struggle / masu form
• 苦しんだ — suffered; struggled / ta form
• 苦しんで — suffering and …; struggling and … / te-form
• 苦しまない — do not suffer / negative form
• 苦しめば — if (someone) suffers / conditional
• 苦しめ・苦しみなさい — suffer!; endure it! (rare) / imperative
• Other common forms
• 苦しみたい — want to suffer (rare nuance; metaphorical) / 〜たい form
• 苦しめる — to cause someone to suffer; torment (transitive partner) / related form
• 苦しめられる — be made to suffer / passive
• More
• 苦しもう — let’s endure; let’s struggle on / 〜よう form
• 苦しんでいる — is suffering; is having a hard time / progressive
• 苦しまなかった — did not suffer / past negative

苦しむ（くるしむ） is a godan verb ending in む. Stem alternates as む → み → ん, giving 苦しむ・苦しみます・苦しんで・苦しんだ. Its transitive counterpart is 苦しめる — “to cause someone to suffer,” used when someone or something brings hardship onto another person.