Learn Hardcore German: Wenn ich unsere alten Fotos sehe, vermisse ich ihn stark und bekomme ein warmes Gefühl. - When I see our old photos, I miss him a lot and get a warm feeling.

Wenn is used for things that can happen repeatedly, generally, or in the present/future: “whenever / when(ever) I look at our old photos…”.
Als is used for a single event in the past: Als ich ihn das erste Mal sah, … – “When I saw him for the first time…”.

So in your sentence, the speaker means “every time / whenever I look at our old photos…”, not one specific moment in the past. That’s why wenn is correct here; als would sound wrong or at least very unusual.

Why does the verb sehe go to the end in Wenn ich unsere alten Fotos sehe?

Wenn is a subordinating conjunction. Subordinating conjunctions in German send the conjugated verb to the end of the clause.
So:

Main clause: Ich sehe unsere alten Fotos. (verb in 2nd position)
Subordinate clause with wenn: Wenn ich unsere alten Fotos sehe (same elements, but sehe moves to the end)

This verb-final order is typical for subordinate clauses introduced by words like wenn, weil, dass, obwohl, etc.

Why is there a comma after the wenn-clause?

In German, a full subordinate clause is almost always separated from the main clause by a comma.

Here, Wenn ich unsere alten Fotos sehe is a complete subordinate clause, and vermisse ich ihn stark und bekomme ein warmes Gefühl is the main clause.
So you must write a comma between them:

Wenn ich unsere alten Fotos sehe, vermisse ich ihn stark und bekomme ein warmes Gefühl.

Why is it ihn and not er in ich vermisse ihn?

Er is the nominative (subject) form of the pronoun “he”.
Ihn is the accusative (direct object) form “him”.

The verb jemanden vermissen (“to miss someone”) takes a direct object in the accusative:

Ich (subject, nominative) vermisse ihn (object, accusative).

If you said ich vermisse er, that would be like saying “I miss he” in English – grammatically wrong.

What is the difference between jemanden vermissen and jemandem fehlen?

They are close in meaning, but the grammar flips the roles of subject and object:

Ich vermisse ihn. – I miss him.
- ich = one who feels the lack
- ihn = person who is missed (accusative)
Er fehlt mir. – I miss him / He is missing to me.
- er = person who is missing (nominative)
- mir = dative experiencer, roughly “to me”

You can often translate both into English as “I miss him”, but in German you can’t mix the patterns.
Wrong would be: Ich fehle ihn or Er vermisst mir.

Why is it unsere alten Fotos and not unsere alte Fotos?

Fotos is plural, and there is a determiner (unsere) in front of the adjective, so the adjective takes the ending -en:

plural with determiner: unsere alten Fotos
plural without determiner: alte Fotos

Unsere already shows plural and “our”, so the adjective alt must use the weak ending -en: alten.
Unsere alte Fotos is ungrammatical in standard German.

Why is it ein warmes Gefühl and not ein warmer Gefühl?

The noun Gefühl is neuter: das Gefühl.

For neuter nouns in the nominative and accusative singular with ein, the adjective ending is -es:

ein warmes Gefühl – correct
ein guter Grund
ein neues Auto

Ein warmer Gefühl would use the masculine adjective ending -er, which doesn’t match the neuter noun, so it’s wrong.

Why does Foto form its plural with -s (Fotos) and not with -en or something else?

Foto is a relatively modern loanword (from Photo / “photo”). Many newer or foreign-sounding nouns in German take -s in the plural:

das Auto → die Autos
das Radio → die Radios
das Foto → die Fotos

Older “native” nouns more often take -e, -er, -n, etc., but Foto follows the “modern loanword” pattern with -s.

Could you also say Wenn ich unsere alten Fotos sehe, ich vermisse ihn stark?

No, that word order is wrong in German. In a main clause, the finite verb must be in second position (the “V2 rule”).

After the comma, you start a main clause, so the verb must come next:

…, vermisse ich ihn stark … – correct (verb first, subject second)
…, ich vermisse ihn stark … – wrong (subject first, verb third)

You can, however, reverse the order of the clauses:

Ich vermisse ihn stark, wenn ich unsere alten Fotos sehe. – also correct.

Can I leave out the second ich and say …, vermisse ihn stark und bekomme ein warmes Gefühl?

Yes, that is perfectly correct:

Wenn ich unsere alten Fotos sehe, vermisse ihn stark und bekomme ein warmes Gefühl.

In coordinated verbs that share the same subject, German often omits the repeated subject, just like English:

“I miss him a lot and get a warm feeling.” (not “I miss him a lot and I get a warm feeling.”)

Including ich (… vermisse ich ihn …) is also correct; it’s just a bit more explicit.

Is stark the usual word to intensify vermisse, and how is it different from sehr?

Both stark and sehr can be used to intensify vermisse:

Ich vermisse ihn sehr.
Ich vermisse ihn stark.

Sehr is the most general “very” and sounds completely neutral.
Stark literally means “strong(ly)” and can sound a bit more emotional or physical, like a “strong feeling”. You’ll also hear other options:

Ich vermisse ihn so sehr.
Ich vermisse ihn furchtbar / schrecklich.

All of these are natural; sehr is the safest, most common choice.

Why are the verbs sehe, vermisse, and bekomme all in the present tense here?

The speaker is describing something that happens regularly or whenever a certain condition is met, not just once in the past. German uses the present tense (Präsens) for:

current situations: Ich vermisse ihn.
general truths and habits: Wenn ich unsere alten Fotos sehe, vermisse ich ihn…

So Wenn ich … sehe, vermisse ich ihn … und bekomme … corresponds to English “When I look … I miss … and get …” – same use of the present tense for repeated / typical events.

Does unsere alten Fotos mean “photos of us when we were younger” or “old photos that belong to us”?

On its own, unsere alten Fotos is ambiguous and can mean either:

old physical photos that we own (maybe of anything), or
old photos of us (from when we were younger).

Often, context makes it clear.
If you specifically want “photos of us”, you can say alte Fotos von uns.
If you want to stress ownership rather than who is pictured, unsere alten Fotos is the natural choice.