What is the ruling on one who vowed not to speak to a certain person, then forgot his vow and spoke to him?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah.
Whoever swears an oath or makes a vow to abstain from something has a choice: if he wishes, he may keep to his vow, or if he wishes, he may offer the expiation of an oath (kaffārat yamīn). The Prophet (peace and blessings be upon him) said: “The expiation for a vow is the expiation for an oath.” Narrated by Muslim. This type of vow is called a nadhr al-lajāj (a vow made in stubbornness or anger).
If a person makes a vow and then forgets it, nothing is due from him, for forgetfulness is among the excuses that lift legal responsibility (taklīf) from a Muslim. This is based on the saying of the Prophet (peace and blessings be upon him): “Actions are judged only by intentions, and every person will have only what he intended.” [Narrated by al-Bukhārī]. There is no intention for the one who forgets or errs. It is also based on the report: “Indeed, Allah has pardoned for my Ummah mistakes, forgetfulness, and what they are compelled to do.” Narrated by Ibn Mājah and others, and graded authentic by Ibn Ḥibbān and al-Ḥākim.
It is stated in ʿUmdat al-Sālik (p. 243), one of the books of the Shāfiʿī school: “Whoever swears, [for example], ‘I will not enter the house,’ and then enters it forgetfully, ignorantly, under compulsion, or being carried, he does not break his oath.” And Allah Almighty knows best.
I can`t afford to get married, what should I do to curb my sexual drives?
You should offer a lot of voluntary fasting, keep busy with useful and permissible acts and make supplication to Allah, The Exalted.
Does the Obligatory Ritual Bath (Ghusl) Suffice in Place of Ablution (Wuḍūʾ)?
All praise is due to Allah, and peace and blessings be upon our master the Messenger of Allah ﷺ.
The more prudent course is to perform ablution (wuḍūʾ) after the ritual bath (ghusl), so as to exit the difference of scholarly opinion and because the sufficiency of the ritual bath in place of ablution is contingent upon a number of conditions and requirements being met. And Allah the Almighty knows best.
Should the Zakah (obligatory charity) giver tell the poor recipient that this is the Zakah of his money?
No, he shouldn`t. But, the Zakah giver should make an intention in his heart that it is the Zakah of his money when paying it to the poor. And Allah Knows Best.