Ο παππούς μου είπε να μη χτυπήσω το καρφί πολύ δυνατά με το σφυρί, γιατί δίπλα είχε άλλη βίδα.

Here μου means my. It is the weak possessive form placed after the noun: ο παππούς μου = my grandfather.

A common point of confusion is that μου can also mean to me. So Greek can have both in the same sentence, for example Ο παππούς μου μου είπε... = My grandfather told me.... In that case, the first μου means my, and the second means to me.

In this sentence, Greek does not explicitly say μου είπε. The person being told is understood from context, and also from χτυπήσω, which is first person singular: I hit.

So Ο παππούς μου είπε να μη χτυπήσω... can naturally mean My grandfather told me not to hit... even without an extra μου.

A fuller, more explicit version would be Ο παππούς μου μου είπε να μη χτυπήσω....

Να introduces a subjunctive clause. It does not have one simple English equivalent.

After verbs like λέω / είπα, θέλω, μπορώ, πρέπει, Greek often uses να + verb. In English, this may come out as to + verb, that + clause, or something similar.

Here, είπε να μη χτυπήσω means something like said/told me not to hit. So να is the marker that introduces the dependent action.

Because δεν and μη are used in different kinds of clauses.

• δεν negates the indicative: ordinary statements of fact
• μη(ν) negates the subjunctive, imperatives, and similar non-indicative forms

Since χτυπήσω is inside a να-clause, Greek uses μη:

• να μη χτυπήσω = that I not hit / not to hit

You would not say να δεν χτυπήσω.

Χτυπήσω is the aorist subjunctive, used after να.

That matters in two ways:

1. It is not past tense here, even though aorist often connects with past forms elsewhere.
2. It presents the action as one whole event, not as something ongoing or repeated.

So να μη χτυπήσω το καρφί means not to hit the nail as a single action or instance.

If the idea were more habitual or continuous, Greek could use να μη χτυπάω instead.

Also, Greek does not need εγώ here, because the ending of χτυπήσω already shows the subject is I.

Δυνατά is an adverb here, modifying the verb χτυπήσω.

• δυνατός = strong, loud, powerful
• δυνατά = strongly, hard, loudly

So χτυπήσω πολύ δυνατά means to hit very hard or too hard, depending on the context and the English phrasing chosen.

This is very common in Greek: a form like δυνατά can function adverbially.

Here με means with, and it introduces the tool used for the action.

So με το σφυρί means with the hammer or using the hammer.

Greek uses με + accusative for this kind of instrumental meaning:

• γράφω με στυλό = I write with a pen
• χτυπάω με το σφυρί = I hit with the hammer

Normally, yes, είχε is the imperfect of έχω and often means had.

But in everyday Greek, έχει / είχε can also be used in an existential way, especially with a place word or location phrase. In that use, it means something like there is / there was.

So δίπλα είχε άλλη βίδα means there was another screw nearby / next to it.

This is very natural in spoken Greek. A more formal style might use υπήρχε instead.

Δίπλα means next to, beside, or more loosely nearby.

Greek often leaves the reference point unstated if it is clear from context. So here δίπλα can mean:

• next to that spot
• next to the nail
• nearby

If Greek wanted to make it fully explicit, it could say something like δίπλα του or δίπλα της, depending on what next to it refers to.

Άλλη βίδα means another screw. Greek can express that idea without an article.

So all of these are possible, with slightly different feel:

• άλλη βίδα = another screw
• μια άλλη βίδα = another screw, with a more explicitly indefinite feel
• η άλλη βίδα = the other screw, a specific one already identified

In your sentence, άλλη βίδα is natural because it introduces a new item that was there nearby.