How does one conjugate verbs in English?

Unlike French or Spanish, which are inflectional languages, English is a syncretic language so its conjugations are pretty flat. With that said, here goes with the verb "to spend":

Infinitive: to spend

Gerund: spending

Present Participle: spending

Past Participle: spent

Imperative: spend (for all persons)

Simple Present Indicative: [Ex. My wife spends too much money.]

• I spend
• You spend
• He/She/It spends
• We spend
• You spend
• They spend

Simple Past Indicative: [Ex. My wife spent too much money at the store yesterday.]

• I spent
• You spent
• He/She/It spent
• We spent
• You spent
• They spent

Simple Present Subjunctive: [Ex. I prefer that my wife not spend money at all.]

• I spend
• You spend
• He/She/ It spend
• We spend
• You spend
• They spend

Simple Past Subjunctive: [Ex. If my wife spent less money, we could buy a new car.]

• I spent
• You spent
• He/She/It spent
• We spent
• You spent
• They spent

Simple Future Indicative: [Ex. My wife will spend a lot of money this week.]

• I shall/will spend
• You will spend
• He/She/It will spend
• We shall/will spend
• You will spend
• They will spend

Simple Future Subjunctive: [If my wife were to spend all of my money, I'd be broke.]

• I were to spend
• You were to spend
• He/She/It were to spend
• We were to spend
• You were to spend
• They were to spend

Present Perfect Indicative: [Ex. My wife has spent all of my money.]

• I have spent
• You have spent
• He/She/It has spent
• We have spent
• You have spent
• They have spent

Present Perfect Subjunctive: [Ex. It's important that my wife have spent no money.]

• I have spent
• You have spent
• He/She/It have spent
• We have spent
• You have spent
• They have spent

Past Perfect Indicative: [Ex. My wife had spent all of my money before I could stop her.]

• I had spent
• You had spent
• He/She/It had spent
• We had spent
• You had spent
• They had spent

Past Perfect Subjunctive: [Ex. If my wife had spent all of my money, I would have killed her.]

• I had spent
• You had spent
• He/She/It had spent
• We had spent
• You had spent
• They had spent

Present Progressive Indicative: [Ex. My wife is spending money like water.]

• I am spending
• You are spending
• He/She/It is spending
• We are spending
• You are spending
• They are spending

Present Progressive Subjunctive: [It's vital that my wife be spending very little.]

• I be spending
• You be spending
• He/She/It be spending
• We be spending
• You be spending
• They be spending

Past Progressive Indicative: [Ex. My wife was spending money like water.]

• I was spending
• You were spending
• He/She/It was spending
• We were spending
• You were spending
• They were spending

Past Progressive Subjunctive: [Ex. If my wife were spending money like water, I'd scream.]

• I were spending
• You were spending
• He/She/It were spending
• We were spending
• You were spending
• They were spending

Present Perfect Progressive Indicative: [Ex. My wife has been spending all of my money for years.]

• I have been spending
• You have been spending
• He/She/It has been spending
• We have been spending
• You have been spending
• They have been spending

Present Perfect Progressive Subjunctive: [Ex. Although my wife have been spending my money like water, I can still say that I love her with all of my heart.]

• I have been spending
• You have been spending
• He/She/It have been spending
• We have been spending
• You have been spending
• They have been spending

See how simple this is? I could write the different tenses all day, but I shan't. It would take too long if I did. Here are examples of the above tenses and other tenses with explanations of what they are:

• John sees them every day. (Simple Present Indicative)
• I demand that he be on time. ( Simple Present Subjunctive)
• He asked me a question. (Simple Past Indicative)
• What if kids ruled the world? (Simple Past Subjunctive)
• I shall give it to him. (Simple Future Indicative)
• If I were to give it to him, it wouldn't do you any good. (Simple Future subjunctive)
• As long as you be living under my roof, you shall do as I say! (Present Progressive Subjunctive)
• He is working right now. (Present Progressive Indicative)
• They have lived there for ten years. (Present Perfect Indicative)
• It's important that he have lived in this state for at least ten years. (Present Perfect Subjunctive)

Some of the ones listed above are very formal English, but you get the idea. None are incorrect, but border on usage. In the end, these are just examples of English conjugations. It's not that difficult.

Anyway, moving on. Here are some examples using some archaic second person pronouns "thou" and "ye" with a cursory explanation of their uses. Technically they're not archaic because everyone in Modern English knows what they mean, but they are no longer in vogue nor are they required in prescriptive grammar. If they were archaic, you wouldn't know what they mean, which can happen when reading Chaucer or Shakespeare. Anyway, here goes:

"Thou" is nominative singular. It later became comparible with the French "tu", which is used to talk to a friend. This meant you wouldn't say this to your boss or your mother-in-law. This was said to a close buddy or someone whom you worked with. "Ye" was the plural nominative of what is today "you" or "you all". It was also a singular "you" when talking to your boss or mother-in-law. Later, "ye" had turned into "you" as the nominative case even before "you" replaced "thou". Since we still use these pronouns in old literature and nothing is older than the Bible itself, without further adieu, here goes:

• Simple Present Indicative: Thou spendest/Ye spend
• Simple Present Subjunctive: Thou spend/Ye spend
• Simple Past Indicative: Thou spentest/Ye spent
• Simple Past Subjunctive: Thou spent/Ye spent
• Present Perfect Indicative: Thou hast spent/Ye have spent
• Present Perfect Subjunctive: Thou have spent/Ye have spent

Okay so "thee" is accusative/dative and "thy/thine" are genitive whereas "you" is accusative/dative and "your/yours" are genitive. Also, for conjugational purposes, there is the old -eth or -th ending for third person singular present indicative ONLY:

• He/She/It spendeth

The reason I am showing this "thou/ye" and -eth ending in 3rd person singular is because we still use this in literature and for rhetorical and humorous effect in Modern English; therefore it's a type of conjugation since "thou" has its own conjugation and the old -eth -th endings for 3rd person singular are still seen as well. This makes them a Modern English conjugation. You may not think so, but look at word conjugations in English from circa AD 1100 and you will agree with me because you won't understand what it even says. You understand this so it is still MODERN ENGLISH. It's just used for effect now rather than common speech.