No tengo fuerza hoy.

Because Spanish often drops subject pronouns when they are not needed.

The verb tengo already tells you the subject is I, so yo is usually unnecessary:

• No tengo fuerza hoy = I don’t have strength today

You can say Yo no tengo fuerza hoy, but that usually adds emphasis, contrast, or clarification:

• Él sí puede, pero yo no tengo fuerza hoy.
  He can, but I don’t have the strength today.

So the version without yo is the most natural neutral sentence.

Tengo is the first-person singular present tense of tener (to have).

The verb tener is irregular:

• yo tengo
• tú tienes
• él / ella tiene
• nosotros tenemos
• vosotros tenéis
• ellos / ellas tienen

So in this sentence:

• No = not
• tengo = I have / I do have
• fuerza = strength
• hoy = today

In standard Spanish, no normally goes directly before the conjugated verb.

So:

• No tengo fuerza = I don’t have strength
• No quiero ir = I don’t want to go
• No puedo = I can’t

That is the normal word order for negation.

Spanish often expresses ideas with tener + noun, where English may use be or another structure.

Here, tener fuerza literally means to have strength, which is also natural in English. But the important point is that Spanish is focusing on possessing strength/energy, not describing your permanent character.

Compare:

• No tengo fuerza hoy = I don’t have strength today / I don’t have the energy today
• No soy fuerte = I’m not strong

These are different ideas:

• tener fuerza = having strength available at the moment
• ser fuerte = being a strong person in general

Yes, both fuerza and fuerzas can appear, but they are not always used in exactly the same way.

• No tengo fuerza hoy sounds like I don’t have strength / energy today
• No tengo fuerzas hoy is also very common and can sound a bit more like I don’t have any strength left

In many everyday situations, both are possible. The singular often feels a bit more abstract, while the plural can suggest your remaining reserves of strength.

Examples:

• No tengo fuerza para levantar esto.
• No tengo fuerzas para seguir.

Both are natural, but fuerzas is especially common with para + infinitive when someone feels physically or emotionally drained.

Because Spanish often omits the article with abstract or uncountable nouns when speaking in a general sense.

So:

• tengo hambre = I am hungry
• tengo sueño = I am sleepy
• tengo paciencia = I have patience
• tengo fuerza = I have strength

If you added an article, the meaning would change or become more specific:

• Tengo la fuerza = I have the strength
  This sounds specific, as if referring to a particular strength already mentioned.

In No tengo fuerza hoy, the idea is general, so no article is needed.

Not necessarily.

Fuerza can refer to:

• physical strength
• energy
• inner strength
• willpower

So depending on context, No tengo fuerza hoy could mean:

• I’m physically weak today
• I don’t have the energy today
• I don’t have the strength to deal with things today

If you want to be more specifically about physical energy, people might also say:

• No tengo energía hoy
• Estoy sin fuerzas hoy

Yes, hoy can move.

These are all possible:

• No tengo fuerza hoy
• Hoy no tengo fuerza
• No tengo hoy fuerza — possible, but less natural in most everyday contexts

The most natural choices are usually:

• No tengo fuerza hoy
• Hoy no tengo fuerza

The difference is mostly emphasis:

• No tengo fuerza hoy = neutral
• Hoy no tengo fuerza = emphasizes today

It is understandable and natural, but depending on the situation, speakers in Spain might also say:

• No tengo fuerzas hoy.
• No tengo energía hoy.
• Estoy cansado / cansada.
• Hoy no puedo con nada.
• No tengo ganas de nada hoy. (if the idea is more emotional or motivational)

So No tengo fuerza hoy is fine, but in real conversation the most common alternative might be No tengo fuerzas hoy or simply Estoy cansado / cansada, depending on what you mean.

They overlap, but they are not identical.

• fuerza = strength, force, inner strength, physical power
• energía = energy

So:

• No tengo fuerza hoy can suggest weakness, lack of power, or not feeling able to do something
• No tengo energía hoy more directly means you feel low-energy or drained

In many situations, either could work, but fuerza often sounds a bit more like strength to act, while energía sounds more like general energy level.

Usually, no—not in the general sense of I don’t have strength today.

La fuerza would normally refer to a specific strength:

• No tengo la fuerza para decirlo
  I don’t have the strength to say it

Even there, Spanish often still prefers No tengo fuerza para decirlo.

So for a general statement, the version without the article is the normal one:

• No tengo fuerza hoy

Yes, the h in Spanish is silent.

So hoy is pronounced roughly like oy in English, though the exact vowel sound is Spanish, not English.

A rough guide for the whole sentence:

• No = noh
• tengo = TEN-go
• fuerza = FWER-tha in most of Spain, or FWER-sa in Latin American pronunciation
• hoy = oy

In Peninsular Spanish, z in fuerza is usually pronounced like the th in think.

Yes.

No tengo fuerza hoy does not change according to the speaker’s gender.

That is because:

• tengo is the same for any speaker who says I have
• fuerza is a noun, and the sentence does not contain an adjective agreeing with the speaker

So both a man and a woman can say:

• No tengo fuerza hoy

By contrast, if you said:

• Estoy cansado — used by a man
• Estoy cansada — used by a woman

then the form would change.